Microsoft abandoned their own browser engine for business reasons. I feel like it should be obvious that what's "worthwhile" for a for-profit organization is very, very different than what's "worthwhile" for a non-profit, individual, or group of open-source developers; "affordable" isn't even in the picture.

This line of reasoning is entirely invalid.

KDE have also abandoned writing their own browser engine. Mozilla continues to produce one but they continue to attract very few outside contributors and rely largely on Google for funding. If no-one's actually doing something we should be wary of blithely assuming that it would be esay or even possible.

Why do we have more than one fork of Linux? Why don't we all just admit that Ubuntu won and call it a day?

The fact that operating systems maintained by small organisations are more viable than web browsers maintained by small organisations should tell you something about the complexity of the web stack.