In general I don't think the marginal benefit of removing support for a certain piece of hardware would be significant in most cases. Most of the kernel is generic across a broad enough spectrum that removing one slice of that spectrum isn't likely to change its breadth.

E.g. there's some stuff like erratum workarounds for old x86 CPUs that would be nice to drop, but they are part of a big framework for handling x86 diversity. Dropping individual workarounds doesn't let you drop the framework.

Exceptions are gonna be cases where dropping the support removed something significant from the lowest common denominator. Big stuff like word size, memory ordering (I assume dropping Alpha would be quite handy), virtual address space limitations.

Sometimes, supporting special use cases like this can be a valuable exercise since it shows you all the places in your project that you made assumptions without even realizing it. It seems plausible to me that supporting niche users improves quality of the project as a whole.