I must say that I share many of your frustrations, although what I'd like is *very* different from what you'd like (i.e.: for instance, while I'm also a JDT user, I don't see that lack of code completion for that specific case as something as important as having a better UI experience -- which for me translates into a good dark theme on all OSes, for which I already provided many improvements even without being a committer) or the lack of a proper macro record/playback engine -- https://bugs.eclipse.org/bugs/show_bug.cgi?id=8519
, which is a request from 2002 with 136 votes (for which I've submitted a pull request and I'm working to make it available in the next Eclipse release -- although I still rely on the time of platform committers, which are also very time-constrained, to put it through).
Also, I think that you're addressing the wrong forum... which is why you're getting answers saying that "it's other people's responsibility" (i.e.: the discussion here should target the IDE as a base for plugins, not about a particular plugin, so, in this sense, having a new editor as a base to target multiple languages makes sense, whereas a bug on JDT should be dealt in JDT -- and while most JDT committers are also Eclipse platform committers, the other way around is not true and they do care for different things).
So, given that you're approaching the wrong forum, it seems reasonable to get such answers... Although I think that even if you approached JDT, given that they have a gazzilion of things going on, the issue you're complaining about may still be low priority (unfortunately, it's reasonable that many tickets open on bugzilla may *never* be done because the number of requests is much higher that the throughput of the JDT team, and until someone steps up -- in the JDT team or not -- it may be indefinitely postponed -- again, this is true for any software projects, not really only Eclipse).
Also, IMHO, what sets Eclipse apart is the fact that it is open source, so, you can do it yourself if you really care about something, whereas when you have some closed-source software, it may be close to impossible -- you can argue that you can pay to have something done, but in practice, that's not true in way too many cases -- try to ask the gmail team to do something you'd like or Microsoft to change something in Windows for instance -- also, if you'd be willing to pay, I'm pretty sure you could find someone who would be willing to implement that for you on JDT (but then again, unfortunately users of open source software seem to expect everything for free, when it actually costs a lot for people actually making the code).