If you are running them 24/7, those old servers (from anybody) are costing you a lot of money. That smart watch running on a solar cell will pay for it's self mighty fast ...
I dunno about a watch but you could probably replace them with a small, fanless Intel Atom Box. I would pay for itself in a few months via the reduced electricity bill.
G5s are actually not that slow. Very, very generalized statement: maybe ~1/4 the processing power per core of a today's mid-range Intel desktop CPU. Still faster than (almost? -- maybe the A53 at 2 GHz can beat them) all ARMs. Of course, their power usage is absolutely ridiculous and was already ridiculous back in the day. Plus, the poor power management either has them make annoying noises or lets them run at 150+ W
idle.
(Edit: ^-- true for any G5, v-- talking about Powermac G5s here)
For todays standards they are pretty loud, back then it was okay I guess. But only because the CPU(s) have truly massive heatsinks sandwiched between no less than
four fans. I have never seen a CPU heatsink that large. Fans included they are the length of a grown man's forearm, about 20x20 cm in cross-section.
And when the fan management thinks it has to turn the fans up... well. Most vacuum cleaners are less noisy. (No irony)
Really the only reason to spring 10-20 USD / EUR for a Powermac G5 is if you have a need to test software on a big-endian architecture, or if you want a difficult to mod case (get a Mac Pro instead, or a proper case FFS).