The issue is indeed temperature related more so than anything else. maintaining a constant ambient temperature. in the sixties (or close to it) will extend the life of either tubes or Ss amps... and they can be left on forever.
Other considerations do play parts though too. As was mentioned, controling power surges & spikes or brownouts is important for sure.
In the military we left much of the electronics on continuously... operating in a very cool ambient temp. I don't recall a tube or SS component failure of any great significance. periodic cleaning and testing of the tubes were the only prerequisites. We tested different tubes at varying intervals. outputs were once a year.
Leaving equipment unattended and powered up indefinitely in one's home however, could well be a problem just waiting to happen. Common sense is required to protect one's investment. We don't all have pockets as deep as the Govt's. I believe that's the variable which forces us to be more conservative thinking when we see the 'always on' OR 'as needed' powering up debate.
If you are going to use it routinely, can control the environments temperature, invest some time to periodic cleaning, and testing of the tubes at given intervals, as well as attend to verying power insurgencies, leaving it or them on is not a problem affecting greatly the life expectancy. Electronics do seem to enjoy a steady state of electricity more so than an intermittent one.
In either case though, just how long a thing will last, or if it will last longer is more predicated upon the topology and build than these other factors.
All that being said I've got a receiver I've had now for 8+ years. For nearly a year it was shelved and kept off. otherwise, I've used it nearly everday, and certainly each week at least. I turn it on as needed. it just went into the service center for a repair. An IC chip was the issue... not caps or transformers. The IC being replaced apparently had something to do with regulating power to whereever and as needed for startup. it was popping shortly after initial power up, and might have damaged the speakers if left alone. Reportedly, it is OK now.
According to some makers of amps, eg., Odyssey, it is recommended they remain on 100% of the time. For the most part mine does. Although I do have to shut it off now and then when reconfiguring other things and it does take more than 30 mins to get back to acting right again following powering down.
BTW.... I've an older PC that is 10 now, and only goes off during a lightening storm. it's working fine, and requires only a clean install of the OS about once a year.