I've been running a 2008 MacBook Pro as a server for 10 years. I've replaced the battery once, about 2 years ago, and one of the fans about six months ago. Other than that, it's been on the whole time.
What OS do/did you run on it? Thinking about repurposing an old 2011 Mac Mini as a server, it should still be more than capable, but since it stopped getting newer macOS versions a few years back it wouldn't get security updates, that kinda worries me.
Just checked, it's actually a mid-2009 MBP and its running 10.11.6 - it's not had a security update since 2018, which yeah, that is a little worrying. Might wipe it and install Ubuntu if there's such a thing as Firewire800 drivers for it.
Using a MacBook Pro 2015 model as a server. Works great.