As you alluded to, many Chromium forks (notable exception is Ungoogled) are backed by tech companies. There's already plenty of intentional changes they maintain in their forks, like Privacy Sandbox,[0] so I don't think preserving support for v2 is a large hurdle for them.
[0]: https://support.brave.app/hc/en-us/articles/10742158329613-W...
Google described Manifest V2 as significant tech debt with new bugs still found there. Either they are lying or it's a non-trivial feature set to continue to support.
So will be interesting to see how many other browsers actually do keep this support alive.
"Backed by tech companies"
Yup, huge red flag. Non-profit is the long-term way to go.