The problem with the add types later approach is that in the end you have half baked .ts files left and right. And it becomes increasingly difficult to know when something compiles if it's because it's correct or because there is an any somewhere.
Yeah, agreed. Ideally this wouldn't happen like that. Temporary solutions like a half-migration have a tendency to stick around for a lot longer than anybody anticipates. But I think it is still better than not having any types at all, if only for having type information in development.