[generationP]

As to decidability: I suspect you mean that Lean's foundations are non-constructive. I'd rather they weren't, but with what Peter Scholze is doing, I'd be surprised if the arguments could be made constructive within just a few years from their conception. This usually takes much longer (e.g., constructive proofs of the Quillen-Suslin theorem appeared in print just a few years ago).

About incompatibility: Last time I checked (some 3 or so years ago), Coq was not backwards compatible either, and libraries had to be ported manually with each update. Sadly, as this is the one greatest anti-feature that is currently putting me off proof assistants, but probably it needs some time, and the quickest way to get there is to maximize usage. From what I know about Coq and Lean, I suspect Lean will stabilize faster than Coq does, due to it being more "declarative" (Coq is based too much on complex tactics, which are hard to make stable by design).