Chuck in Pokemon Gold or Silver into that GB and see how good they work today, I mean... the game will boot but the internal clock will have made the carts effectively worthless for save files.
It's the same with resi evil 7, that on switch wasn't a title I'd ever expect to be able to play on the system, but now you can (if you're in Japan) and enjoy playing the game for 6 months for 20e, which is cheaper than other formats and the game is far far far shorter than 6 months worth of play time, but you can finish it for 2/3 the price it is on the other systems with no download required, think of them as options for people to enjoy the games rather than "scamming"
Using the battery of old cartridges is a really bad comparison. Back then, that tech was the best they could do to preserve the saved files, now modern cartridges can save way more info for a longer period of time. And even then, I can just add another battery to my GC cartridge and play just fine, but if I pay for RE7 and I want to play after a year I have nothing. Even most digital games, I can just save the games I purchased as a backup. I can't do a thing with cloud gaming.
If you have no issues doing the soldering work to remove and replace the battery in a GB cart then you should equally have no issues with getting a version of a digital download game to work on a platform 10 years after it has come out, considering that board modification of a GB breaks the EULA with it, the same can be said about modifying a console to play "backups" of games anyway.
As for the resi evil 7 thing... if you pay for a 6 month lease to play it... after 6 months you have nothing if you want to play it, that is the nature of a 6 month/180 day lease to play the game, it works the same if you buy insurance for your car for 6 months, in 12 months time you will have nothing because that is how time and leases work, the idea is you buy it, then play it during the purchased time.
just incase the obviously bad analogy of insurance annoys since it's not gaming related
psn + subs
You aren't meant to buy those things if you aren't going to use them for 10 years, you buy them, and use them, as a consumable.