Just some early draft thoughts
Increased modularity is a long-standing request of jQuery. 3.0 should deliver on that. Many authors will use jQuery 3.0 in ES6 codebases, and often alongside frameworks that have overlap in functionality.
That said, a majority of developers will use a monolithic build. jQuery 3.0 must aggressively remove API surface area and functionality from this core build to protect developers from performance footguns. The jQuery team has reasonably voiced concern that removing modules means it's less likely for people to upgrade. While some users do attempt jQuery version upgrades, many freeze their version and never revisit it. I'd like to get more data on this phenomenon as I think we're optimizing API deprecation policy for a small audience.