We spoke to folks who had built browser on top of webkit and they spent nearly 2years just fixing random bugs and getting sites to work. I'm sure firefox/gecko engine would probably work better than webkit, but the point still is: if we don't use chromium, a lot of work does have go into fixing website compatibility issues, adding support for extension. We're 2 person startup and chromium codebase was easier to build on top of and provides a solid baseline.
And Brave has shown that you can build a privacy-focused browser on top of chromium still.
In the agentic browser era, I think there are so many low hanging fruits on privacy which are more important to address -- sending all your sensitive data to Perplexity Comet to sell ads is a pretty bad option right now. Supporting local LLM, letting folks bring their own API keys is crucial.