In my view, ETL is really two things. First, it is an extraction and data cleanup process—essentially liberating data locked up in a variety of systems in the organization and removing an system-specific non-sense. Secondly, that data is restructured for data warehousing queries (i.e. made to fit the type system of a relational DB, forced into a star or snowflake schema, perhaps broken up into a high performance column format, etc). Conflating these two things is a problem. The clean, integrated repository of data should be available in real-time as well for low-latency processing as well as indexing in other real-time storage systems.
I think this has the added benefit of making data warehousing ETL much more organizationally scalable. The classic problem of the data warehouse team is that they are responsible for collecting and cleaning all the data generated by every other team in the organization. The incentives are not aligned: data producers are often not very aware of the use of the data in the data warehouse and end up creating data that is hard to extract or requires heavy, hard to scale transformation to get into usable form. Of course, the central team never quite manages to scale to match the pace of the rest of the organization, so data coverage is always spotty, data flow is fragile, and changes are slow.
A better approach is to have a central pipeline, the log, with a well defined API for adding data. The responsibility of integrating with this pipeline and providing a clean, well-structured data feed lies with the producer of this data feed. This means that as part of their system design and implementation they must consider the problem of getting data out and into a well structured form for delivery to the central pipeline. The addition of new storage systems is of no consequence to the data warehouse team as they have a central point of integration. The data warehouse team handles only the simpler problem of loading structured feeds of data from the central log and carrying out transformation specific to their system.