I think the tricky thing here is that I simply don't have the time, patience, or resources to maintain this stuff. Let's say I have a LE-only project. Someone ports it to work on BE. Now it needs ci for BE. I write a patch in the future and the BE tests fail. Now I need to fix them. Potential contributors need to get the tests to pass. Who's using BE, though? Is the original porter even still using it?

The author betrays their own point with the anecdote about 586 support: they had tests, the tests passed, but the emulator was buggy, masking the issue. Frankly, if you're the Linux kernel and nobody has the hardware to run the tests on an actual device, it says a lot. But it also shows that qemu is struggling to make it work if the emulation isn't working as it should. How is someone who runs a small project supposed to debug a BE issue when you might have to debug the emulator when a user report comes in?

For me, I'll always welcome folks engaging with my work. But I'll be hesitant to take on maintenance of anything that takes my attention away from delivering value to the overwhelming majority of my users, especially if the value of the effort disappears over time (e.g., because nobody is making those CPUs anymore).