Linux

This is incorrect. It's true that most (in fact, I would say almost all) forks go nowhere but that doesn't mean forking isn't incredibly valuable. Even the example you cite, "original project is dead" isn't just incidentally useful, it's critical to open source. Other examples include:

• project's core team is part of a for profit org that is moving the project in a bad, profit motivated direction:
• project's leader suddenly and dramatically loses respect (maybe he killed his wife or something);
• project's leader dies without leaving a digital will regarding who controls the core repo;
• project continues to direct effort into features while falling to address major security concerns;
• project is healthy and useful in every way but there is an important use case not being addressed, and the fork would address it.

Even if 99% of forks fail, that's irrelevant because 99% of original projects fail in the same ways. Forks are critical to open source.

So mass adoption is your answer, and I'd say you're misguided. The purpose of FOSS isn't to make a profit, but to satisfy uses and needs. If a few people have a need for a fork and use it, then it's a success.

You're judging FOSS software by popularity, rather than use, as though it's for profit.

Most new businesses fail as well. Maybe we shouldn't be starting new businesses either? Or perhaps this more about people being unprepared and out of their depth whether it's starting a new business or forking a code base. And not the individual actions themselves.

plenty of Firefox forks are still thriving and getting updates

All the Firefox forks are pretty much dead as well.

Firedragon and LibreWolf seem to be pretty healthy. I've been using LW daily for over a year and FD daily for 1-2 years before that.