Between Microsoft on one hand and all the other browser companies (Opera, Brave etc) on the other, I wouldn't be so sure.

Following Google's lead is massively cheaper than having a custom fork or engine, but it's not impossible. If Google does something wild and unpalatable to all these companies, I wouldn't be surprised if we see a Linux Foundation version of Chromium, with custom skins on top.

That could even get Mozilla to jump on the bandwagon, especially if they lose 9x percent of their revenue due to Google's antitrust remedies, which is not that unlikely.