I think you should look for very large projects. Small projects, however popular, can get replaced by new competitors in no time flat. You also want ones that

I think you should look for very large projects. Small projects, however popular, can get replaced by new competitors in no time flat. You also want ones that already are decades old. Those have proven to be able to live for decades, and (as far as we know) it’s not like projects can die of old age. So, things like Linux, PostgreSQL, gcc, or LLVM (maybe, because it’s not yet 20 years old).