i think you have to consider an older computer to be a part of the printer... just to print
This is a good plan if a person already has the "old" computer like I do or if some software will only run on it. As far as I can tell, my Studioprint RIP still only runs on Windows but my system works perfectly:
"old" PC built new back when my epson 9800 was the newest thing. The PC was built solely for the purpose of running the rip and nothing else and is connected via ethernet to my macs over the years. The PC runs Windows XP which is also hassle-free and stable. It is not connected to the internet so there is no virus worry and the whole thing is as if it's still 2005 ;-)
As for older macs slowing on larger digital files.... my 2010 mac pro 6 core 3.33 was upgraded to 2012 12 core 3.46 a while back, has 96gb ram, separate ssd's for OS and a working space both mounted via pci directly on the board so it's actually still quite fast enough and I'm even now working on 200gb raw files from a Fuji GFX 100s.
Capture One 22 also runs perfectly so far in the Mojave OS which is the last OS this era mac pro can use, i think. With C1 22 and adobe cloud working fine on it, I'm good for a few more years at least.