Manifest v3 has an upper limit of rules that can be implemented, so if you don't see how supporting it is a problem, you're blind.
Supporting v3 and v2 inevitably leads to v2 support being dropped. Firefox has done this plenty in the past with the goal of "cleaning up the code base". Taking care of 1 code path > Taking care of 2 code paths.
It's simple on the face of it, and all results in less control for web users.