In this case though literally what are you even complaining about? If it's physically isolated so you don't need to update the old hardware, then you don't need to update OpenSSH either. Right? You can just leave whatever terminal you use right now on whatever it runs right now forever.

That's the basic discontinuity here. The reason one would want to keep OpenSSH updated and continuing to receive development work is for use in a networked environment without full physical security from the world. If you have something stuck on DSA it shouldn't have any exposure to the world. So there's no problem here. At the very least one could softgap with old-ssh-in-VM that is completely restricted in what it can access.