That said, there is a bright side of a browser engine monoculture: JavaScript will finally meet its very long-overdue demise. Google could implement Dart or an evolution of that language (they tried this around 2010, but Blink wasn't on 95% of [desktop] computers back then) and kill off JS overnight, since if every browser supports it by default, there's no reason not to use it over JS.
The massive downside, of course, is that Google also has the ability to design its browser so that extensions that perform adblocking can't run, and that'll be everywhere too- though I suspect that the number of people who would pay the price of a VPN connection a month for a modern browser with guaranteed ad-blocking support is quite large provided there's no other choice.
So yeah, we wouldn’t get a new browser engine, but I think a new “fully featured” browser imho is possible. Vivaldi for example, while built on Chromium, is substantially different in terms of features.
I have been using Vivaldi Browser for the past four years. It's the most feature-rich browser I found and the team behind it is trustworthy. Their privacy policy specifically says they do not track me or collect information on my navigation history.
But although I've not used Firefox in years, I still don't want to see it die.