That's basically exactly what Brave (the only Chromium browser still supporting uBO) is doing to support uBlock Origin. But it's flimsy and likely has an expiration date.
My point is that when MV3 was announced there was an assumption that maintaining a Chromium fork to continue supporting MV2 would be too much work and too expensive. But now with LLMs that's no longer a safe assumption. The non-Chrome chromium based browsers can fork chromium and add back MV2. No need for any expiration date.
We should prepare for a future without Firefox, because one day it won't be there anymore.