Hmm. I think that a better way to do it is probably in the OS, rather than in-game, on a per-game basis.
Processors thermal-throttle today, and OSes can limit what modes they’re allowed to shift into. And my guess is that usually, if someone wants to constrain performance, they want to do it systemwide, rather than for an individual game.
On the game developer end, if the player wants to play both in a performance-limited and not-performance-limited mode, I’d think that there are probably two ways to go about that:
Permit for two different sets of saved video settings, where the player can flip between them. Honestly, I think that this is probably more tweaking than most players are going to do.
Provide some kind of adaptive quality mechanism. Then, if the computer becomes “lower end”, then the adaptive quality system just twiddles settings until the target framerate is maintained.
There’s also a third point you make here, and that is that in a world with battery-powered devices, CPU/GPU usage actually matters. It’s not zero-cost to just use whatever’s available. I remember submitting an issue some time back for Caves of Qud, where the thing ran a busy loop when the window didn’t have focus, even though the game was paused (which the dev fixed, kudos to them). I noticed it because the fans would spool up when the game was in the background. That’s a game that, because it’s turn-based, has the potential to use very little CPU time, even when the game is in the foreground.
I think that there’s a fair argument that historically, most game developers, aside from maybe mobile or portable console guys, haven’t needed to worry much about consuming resources if they were available.
Speaking as a player, though, I don’t much care about power consumption if a system has wall power. But I care a lot about it if it’s battery-powered.
For phones, I kind of wish that Google would consider providing a “battery usage” rating in the app store that provides some kind of approximate metric for how much CPU time the game uses while active – if Google is going to send all kinds of telemetry from devices, might as well use that for something useful. Maybe permit the game developer to register multiple “modes” (high-power, low-power) and give a ranking for each. As things stand, though, there’s no way for the potential customer to know power consumption, and this would help push that information out to the customer.