Do we really need the latest kernel on them, though? As in, is it worth it?

In 100 years, if we need to run kernel 6.12 to use a 3c509, is that all that bad? It's not like being "stuck" on an old IRIX without the proprietary dev CD where you have a catch 22 problem.

An old system version that's completely open is whatever the sysadmin version of "turning complete".

Sure, port the open system to the hardware that does not have an open system. But I question the value of keeping it up to date.

All else being equal, sure it'd be cool. But all else is not equal. So yes, ripping out old code is the right move for Linux, in my opinion.