why go along with Google?
It's a matter of self defense.
Since manifest v3 has a lot of API changes, extension developers who port to it will be under pressure to drop v2 support. New extensions developed after v3 is widespread and fully operational will of course target the gigantic Chrome market, ignoring or only addressing as a lagging afterthought the relatively tiny v2 market.
Browsers which do not support v3 will be left behind in a shrinking pool of sad remnant extensions. Look at the state of extensions for Firefox forks which haven't picked up WebExtensions (manifest v2) support.
None of this would apply if it were truly just a different manifest format; if a single extension could easily be packaged in either 'v2' or 'v3' format with a few simple build procedure changes. The name is misleading.
Biting the hand that feeds IT
