Sounds like a way to prevent having to completely rewrite from the ground-up to keep up with the web a la angularjs.
I like React because it's a lot smaller but I totally see the advantages of Angular 2 and I kinda dig the major release cycle.
As soon as you rely on packages that support Angular 2, the 'good and ready' argument breaks, since you can't start till all the packages you rely upon have already made the upgrade, and you can't take to long either, because you can't expect further work or fixes to be made to older versions of the packages.
12-month breaking changes, that would have made sense.
That said, breaking changes every six months seems to be commonplace in major open source packages. My initial reaction was 'OMG, now they want to repeat the Angular 2 debacle every 6 months?!?', which doesn't seem to be the case.