Why would it let smoke out? I doubt a C64 has any power management. So whether it idles or boots Linux via a couple of emulation layers, the thermal load will be exactly the same.
I know there can be issues with thermal saturation on a heatsink design-- it was expected not just to generate N watts of heat, but to only do so for M hours. Can you expect to leave a real 64 on for days or weeks and it will stay up? I wonder if stores that had them as demo units when it was a relevant product, for example, power-cycled them regularly.
I know from experience if you block the bottom intake vents on a VIC 20, so it can't convect properly, it will eventually start acting funky.
On a related note, I understand the 64's original power bricks are considered timebombs, they might also not appreciate being left on for weeks at a time.
Can you expect to leave a real 64 on for days or weeks and it will stay up?
Yes. These early computers found their way into various embedded control applications too, and I suspect there's quite a few C64s still in operation that way; they would've been replaced long ago if they weren't stable. An article occasionally appears when someone discovers this:
A fairly simple diagnostic test for a C64 is to touch the chips - if the machine is in working order the chips will be warm to the touch but not uncomfortably so. If any are uncomfortably warm odds are they've failed.
If you block all air circulation, sure you might eventually end up with problems, but it takes quite a bit.
I just saw a museum exhibition that featured a C64C, running all day, with a dust cover on, on what appeared to be the original PSU. I think it makes sense to be cautious about your own unit, but they’re probably not as vulnerable as people assume.
Definitely the Achilles heel in any C64 setup. Perhaps new PSUs are visually distinct from the old ones, but practically everyone had to replace theirs as they aged.
Now the 64C was released in 1986, four years after the 64 and its faulty power supplies came out. I don't know whether Commodore had decisively fixed the flawed PSUs by that time, but I know for sure that my second PSU lasted for the lifetime of that device too.
Just because it's likely an old and long dormant piece of electronics, nothing to do with linux beyond it having to run the machine for multiple days 24x7. My understanding is they don't come out of deep dusty storage in ready for service condition. Leaky caps.
C64 caps are generally fine and do not need replacing. The most common things to spontaneously go bad of ”old age” on a 64 are probably RAM chips and the PLA, and of course the power supply is a time bomb.