Well, there are always different ways for devs to choose. Right now users want mv2 extensions, but later they could switch to mv3, so devs could decide to drop mv2 support.

I agree with extensions store, if google removes mv2 extensions from their store that will make harder to install them, it will also would be not good for devs probably, so eventually mv3 will take place… Or Firefox could be a place where all mv2 are now…?