Real time clock doesn't matter, I thought all Gameboy games had battery-backed saves. I can find forum posts of people talking about replacing batteries in their Red cartridges, troubleshooting why the game still won't save after replacing them, across iFixit, Reddit, GameFAQs. I'm pretty sure that G/S/C only took additional advantage of the existing battery, and didn't introduce an additional battery.

The RTC constantly ate power way more than the battery-backed RAM, that absolutely matters.

It makes a difference, absolutely, but the battery still exists and it's surprising for it to live for so long.