Consider that KHTML, the grandfather of just about every modern engine that isn't Gecko, was a fairly small open source project. Ladybird is open source and surprisingly usable for its youth and single developer.

Speaking from personal experience, I write my best code when I'm frustrated by the absence of something I need. I'm sure many others here are the same way. If a sufficient fire is lit under the community to make the web continue to work in the absence of the old guard, I'm pretty sure it'll happen.