Seems like one issue would be that an older Mac device is not going to be able to run new software updates once it ages out and wont get new security updates once that happens.
That being said, if you use the Mac as a server, behind a NAT/Firewall with only some ports open, use up to date server software, and don't use it to surf the web, then the security impact of using an outdated OS is minimal.
You could of course also just install Linux on it.