I think that this is still an issue - certainly caused chaos during an important meeting yesterday!
The problem is that it is quite common to need multiple people have edit access on a shared edits post, so the Wiki + Shared edits combo is highly useful.
Also, it is quite common to want to ‘upgrade’ a wiki post to a Shared Edits post for brief periods of intense synchronous activity. Personally, I think this is the best way to think of it, and the UI should match this - i.e. Shared Edits is an extension of wiki functionality, not an alternative.
Or perhaps Shared Edits could simply include edit access to the post as part of the package, and it becomes an either/or (with both impossible to select). I can’t really see why that would cause problems.