I've got a 2002 mirrored door G4, twin CPU 1.0GHz, that has never missed a beat.
I have a Mac Pro too, and obviously, it'll last much the same - until its performance or power consumption becomes un-productive.
I'd also have suggested that computers with rotating disks - ie Winchester drives - would be less reliable. But unfortunately the solid state ones despite having no moving parts do have evidently limited lives. I'd have thought a computer with no fan, no moving parts, and with a motherboard designed for long life, would be the most reliable long term. However the only ones like that have batteries, and they sure do not last.
Dust is the enemy of computers, and once, dry solder also was a problem. I think dust would still be a heat killer, not sure that today's modern thin production methods are subject to the old wear and tear.
We have 2006 white 24" iMac - which has the best screen of them all because its fonts are the correct size and its anti-glare - has suffered from heat issues. The hard disk became un reliable, and one had to run the fans on full speed to stop the screen from over heating and the image being interfered with. With perhaps a new fan, new hard disk or SSD, and a new thermocouple to tell the temperature, it would be reliable once again. Worth doing? Not sure ... and because its a bit sick, it doesn't feel right to sell it either. So I put iMac at the bottom of the reliability list.
We have also an ex school (three years at school) white plastic case Macbook, that is hmmm ... I guess 5 years old now. Its had a tough life, but ignoring a 3rd party battery that only lasted 8 months, its been bulletproof, except for a design floor in its headphone jack that Apple had to repair (the sound out did not work and Apple replaced the motherboard). The trim was flakey too around the keyboard area - Apple replaced that too. And around the screen. So although I say its been bulletproof ... it needed its warranty. But perhaps a current Macbook Pro that was used as a desktop, would be very reliable. As they are somewhat ruggedized, so sitting still, without using their screens or keyboards, they'd last forever IMO. But also, that would be a waste of money!
Price / Performance, I would not be surprised if a twin disk server orientated Mac Mini wouldn't be the best buy. Don't buy the warranty, instead buy some shares with the warranty cost, and buy some shares with the money you save from buying something more expensive. And when the mac mini fails, you'll have lots of money for a fancy machine from selling the shares.