Here's some alternatives that use the same base as Mozilla, and maybe they might pick up the shards soon: Waterfox, LibreWolf, Mullvad Browser
Considering Manifest V3 in Chromium and how the browser kit is forcing quite a few things on us there's also a solid chance people will independently rework the chromium base into a new web browser toolkit soon, but we'll have to see. Either way, next few years will undoubtedly bring a lot of change to the browser landscape.
A browser tied to a VPN provider would probably harm adoption in the wider scheme of things. That would be a next-to-impossible sell for business IT for one thing, but also the optics generally aren't great at all, especially if said VPN provider finds itself in hot water related to the primary usecase for commercial VPNs (illegal activity)
Wait, did I miss something? I was under the impression that Vivaldi wouldn't be affected by the Manifest v3 change since their adblocker is independently developed... is that not the case?