If they hadn't deprecated the old extensions yet, the newest version would be pretty much the same as using the old version. This way, some people who don't rely on those extensions (I believe half of the users don't even run an extension at all?) can benefit from those improvements now, and you'll be able to do so later.
That's not true, the legacy extensions kept working w/ the project quantum stuff until mozilla broke completely unrelated stuff. E.g. Greasemonkey only broke because they renamed (not removed) a few things.
