I think the question of my GP is: Aren't better new kernels better for most people than slightly worse new kernels that are better for only very, very few people? :)

If your point is that a.out support makes kernels worse for everyone else, you can simply compile it out and it will not affect you.

The issue is keeping the code in tree. Its inclusion doesn’t affect the quality of the kernel, its just a maintenance burden.

In what way specifically are they better for the use case of compiling Atari Jaguar software using decades old toolchain?

If I want to run these binaries on my daily driver, it’s much easier if I don’t have to start up a VM. I want the latest Linux kernel on my daily driver for hopefully self-evident reasons.