I think you're underestimating the size of the demographic who care about FOSS, or who believe FOSS to be more trustworthy. It's not big globally, but I think it has a lot of overlap with people who use Firefox and people who care about online privacy.
There's no way that the intersection of people who care about online privacy, don't mind that Firefox is less web-compatible than a Chromium-fork, but also don't care at all about FOSS is big enough to sell a product that everyone else is providing for free, especially since you're not just saying goodbye to the Googlebux, you're also saying goodbye to part of your workforce (the volunteers,) free advertising (FOSS advocates and default installs on desktop Linux,) web devs (no one would pay to test pages on a niche proprietary engine,) extensions (most high-profile extension devs are FOSS advocates,) and most of the community. It wouldn't work.