What you mention on the UI front is why I've been leaning more towards just using a nice base like bootstrap, and building from there. Once you are reliant on a full-featured toolkit, working on edge cases becomes much more complicated, not to mention maintenance and support. Things have been evolving fast since very early on... it's nowhere near as bad as the v4 browser days (IE4 & NN4, not HTML4)... Things will get more diverse, but I do think we will see at least some consolidation and reduction in popularity in the next 5 years, as many apps using the various frameworks start to retire.

Personally, I would like to see jQueryUI's widgets ported to use Bootstrap as a base theme. I think the interfaces for it are decent from the JS side, but the markup leaves little to be desired.