I agree. 100% Chromium-based browser market share would be OK because Chromium is free software and the ability to fork is real. But I think what really matters is not at the level of browser code, but getting builds of that code that act as user agents (i.e. in the interests of their direct users) into the hands of the multitudes.
Firefox is probably the best we have at that now, and it’s deeply imperfect, but the imperfections have nothing to do with the technical implementation of the browser and everything to do with the imperfections of Mozilla, and the ways that Chrome and lesser proprietary browsers are far worse than Firefox as user agents have nothing to do with their technical implementations and everything to do with their sponsors’ far worse imperfections than Mozilla.