The issue is around maintenance/development burden. For example, when manifest V2 was dropped in favour of manifest V3 it is possible for a downstream project (Edge, etc.) to maintain V2 support. However, that gets harder the further along the projects go and the code diverges; that may mean keeping more code around (if interfaces or utility classes are changed/removed), or rewriting the support if the logic changes (such as the network stack).
It's like projects trying to keep Firefox XUL alive, or GTK+ 2 or 3.
The project has now moved from just updating the external dependency to working on that and possibly actively fighting against the tide. That is a lot harder and requires more work each time you update the dependency.
So in effect you have vendor lock-in. And if the vendor controls or affects downstream products like plugin developers (targeting manifest V3) or application developers (targeting GTK+ 3 or 4) then its even harder to maintain support for the other functionality.
It's like projects trying to keep Firefox XUL alive, or GTK+ 2 or 3.
The project has now moved from just updating the external dependency to working on that and possibly actively fighting against the tide. That is a lot harder and requires more work each time you update the dependency.
So in effect you have vendor lock-in. And if the vendor controls or affects downstream products like plugin developers (targeting manifest V3) or application developers (targeting GTK+ 3 or 4) then its even harder to maintain support for the other functionality.