Swift devs keep saying it'll get less fragile, then they rewrite "exceptions", or how strings work, or anything else and all your code has to be fixed line-by-line. I do not believe it'll be stable this decade.

Xcode stops supporting old features in XIB, or a class has a deprecated and then removed property, and the deserialization fails. XIB is not XML with a nice semantic DTD, it's an object serialization graph that happens to use XML.