Having worked on Glimmer 2 myself, I don't think it's accurate to say that Ember devs worked on it for 14 months (that is, it took 14 months of work to same 100ms). It had initial work done for a long time and then it sat for a while unchanged because it's an open-source project that people work on when they have time. Month later work started up again to get it out the door.

I think that since LinkedIn is "all in" on Ember it will help to get some of the cruft removed and things moving forward. LinkedIn has probably the largest and most complex Ember app in existence, so there are a lot of learnings to take an use to improve the framework and ecosystem.