Yes i know, in fact i thought about it when writing that line but i thought it'd be obvious that the issue still remains, especially if said files are too big. It might be easier but you are not really solving the problem as well as opting in to the new functionality when and if needed.
I hope we can agree that Python 3 was qualitatively different because all of your libraries had to migrate first.
Now, your new argument depends on how hard it is to migrate a single file. If that’s sufficiently easy, you simply do it. Migrating to the last Rust edition, I had to fix maybe a handful of things in an entire crate; most files didn’t have to be touched at all.
Certainly, Python 2 to Python 3 was harder due to all the dependencies and having to convert the entire program.
My argument isn't really about migration is about not having to migrate if you don't need the features while not losing anything - by opting in to the new features/changes you can use whatever you want without wasting time on changing your code to do the same stuff just in a different way (unless you decide that this different way is actually better and worth doing, but that'd be your choice and not something indirectly forced on you by the ecosystem you decided at an earlier point in time to rely on).
