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).
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).