> My Photoshop install from a previous life is a major reason I still keep a 15-year-old Mac running.

I've debated setting up an offline "creative corner" machine with a comfy older OS and Photoshop, something like Snow Leopard and Photoshop CS1/CS2, precisely because the core of what makes PS useful for me hasn't changed in ages. For my use case the new features added in versions since simply aren't worth the bloat and slowness that's come with them.