So, why the current narrative around ETFs, futures and similar is wrong, I believe:
Start with the idea of Darwinian natural selection. People say "evolution is a theory, it isn't proven", now you may think they're stupid because the theory is proven, but the error is much more fundamental: natural selection is not really a theory at all. It's an inevitable logical consequence of a simple set of conditions: basically, reproduction through encoding along with random variation. Natural selection has to happen in this case.
There is a similar story around the famous "Efficient Market Hypothesis". People love to say that this theory is wrong, in fact I get the impression many think they're very smart for realizing that this is not how the world works. Totally wrong thinking, for the same reason: if there is a (even somewhat free) market, and information reaches any participants about a news event Y that will lead to the value of Y being higher than its current market price Z, they will buy it now, and price will be >Z now, not later when Y occurs; and if they don't, someone else will. Price reflects the current state of knowledge of the market, of course not exactly, and of course the state of knowledge of the market is not total, some facts are not known to anybody. There are plenty of caveats and subtleties (see the weak, strong and semi-strong forms of EMH; note that certain "famous Bitcoin commentators" who have wiseacred about EMH for literally years never even mentioned these three forms, showing themselves up as completely ignorant of the thing they claim to champion ... sidetrack), but that doesn't alter the fact that this mechanism is a logical inevitability, not a 'theory', given certain very obviously correct assumptions.
(By the way I have an idea around these thinking errors that I like to call 'the fallacy of agency', but I'll leave that for another day).
Now consider the gold market. There is a certain, very large class of commentators who have been opining for decades that the price is 'suppressed'. Consider what that means in the context of EMH (which we are pointing out is logically inevitably correct to some vague approximation). Suppose there were a real, objectively correct price of gold in dollars which is 10x its current price. Then the only effect of a fake, suppressed price in an ETF is either (a) absolutely nothing if nobody can ever convert those paper claims into real gold or (b) a cause for jubilation if there is any way to convert those certificates into real gold, as you then have a truly spectacular arbitrage opportunity. Note of course that there are gold futures (on the CME, I believe), and like other commodities futures products they are cash settled, not settled into the physical, so you might think, again: no arbitrage!, but think: if cash is convertible into and out of gold anywhere then the arbitrage will still exist, albeit it may be pretty costly. If you think a 10x disparity between the underlying physical and any derivative is going to persist if there is any route between the derivative and the underlying (via cash), then you are just delusional.
Now all of the above was based on the initial premise 'suppose there were a real, objectively correct price of gold in dollars', and there, I believe, lies the rub. Especially for monetary instruments, talking about 'objective price' (a bit like 'intrinsic value') just opens up a world of argument (see below). So let's take the opposite case: suppose the price is purely subjective and purely a matter of sentiment. Then, in that case, you can make an argument for 'price suppression' specifically in the sense that a certain elite class, in control of the mechanisms of easy access to derivative instruments (like ETFs, futures) can effectively control public opinion through market price publication. But if that's the true situation, what exactly is the point of any asset that can be thus controlled? It's basically worthless and rubbish as a form of money.
I know that many people are aligned somewhat with the above paragraph. But I think it's the previous paragraph that is more pertinent.
Another common narrative (and I'm finally getting onto Bitcoin here, but gold also) is that "there is never such a thing as intrinsic value (objective price), all value is relative", and people justify the idea of Bitcoin as money based on this, but I think this is wrong, also. Gold obviously has some intrinsic value, although it's debatable what form it takes; its non-perishability and especially non-corrosion (shiny) properties really matter, for example, aside from more obscure features like conductance. But there is an overlap between 'intrinsically valuable properties' and 'properties which make it a good money' here, so the two get a bit conflated. You'll sometimes hear people say 'Gold is $1K but only $100 is intrinsic value, so it's OK that Bitcoin is $0 intrinsic value' but I see this as hopelessly confused. First, gold's intrinsic values are partly monetary, so you can't make that split (non-perishability/non-corrosion, amongst others). Second, Bitcoin does have what can be called 'intrinsic monetary properties', for example, easy high confidence verifiability, censorship resistance, etc. They are properties specifically aligned with being a monetary good, for the obvious reason that that's what the system was designed for.
So given this background, we can kind-of talk about 'objective price', while at the same time totally understanding that there is a huge amount of sentiment, speculation and subjectivity about the actual price we see in the market, for gold or Bitcoin. In the case of Bitcoin in particular, you could almost assess the price as having three components: the 'intrinsic' monetary component, the sentiment/speculative component and a third: the projection of success into the future (because unlike gold, it's a new form of money and the market is still trying to predict the level of success it will have in the future).
Now introduce the idea of a Bitcoin ETF, or a cash-settled future market (which we already have). We also btw have things like options, although the market is much, much smaller than that for gold. Are these derivatives a danger to Bitcoin itself?
In my opinion, not really. You can certainly try to manipulate prices, but as detailed above, the limitations are pretty clear: (a) there will be cash-based arbitrages from, say, the Bitcoin futures market through to other exchanges and P2P, OTC trade. These arbitrages will generally be much easier in Bitcoin than gold, so while I have somewhat dismissed the idea of price suppression in gold (the arbitrage is still there, even if difficult), I pretty much totally dismiss it in the case of Bitcoin (it's just way too easy to perform these arbitrages if you are a serious operator).
My only caveat to the above is if we somehow have a global police state controlling cash to the extent that nobody can transfer fiat money without being 100% surveilled. I'm not as worried about this as some because I don't think national governments will ever cooperate enough.
What's left is, can the effect be one of psychological manipulation? Sure, as with gold, that's possible, and if ultimately the independent value of BTC to people is basically nothing, and it's all speculation/ sentiment driven pricing then of course it can be manipulated (see: advertising). But in this case it's genuinely worthless and nobody should care.
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I'd like to point out what the above did not discuss: is it safe to have, say, a Bitcoin ETF? I mean, safe for people that invest. I think the answer may well be no, fundamentally because it's a digital bearer asset, and that is exceptionally dangerous to concentrate. In the past I have called Bitcoin "financial plutonium", in the sense that it's very dangerous to ordinary people because they don't really understand what they're dealing with.
However we already have plenty of people acting recklessly by storing BTC on exchanges and other similarly risky endeavours. I don't think this is different, except perhaps in terms of scale.
Risks raised by people like Caitlin Long are also in this bucket imo; there may be a risk to the actors involved, but not to Bitcoin itself as a system.