The MIG cannot do any of that "other stuff". Its only use case is piracy (technically, its only dubiously-legal use case is backing up your owned cartridges). The only code it can cause the Switch to execute is signed, official Nintendo code, exclusively when paired with a valid cartridge key. It can't be used for music playback or emulation or alternate desktops or as an entry point for custom firmware or for homebrew applications. It can't even load game updates or DLC. Your memory of DS carts is irrelevant, the MIG is a completely different type of product.

RCM on the Switch enables piracy (of dumped games from carts) and homebrew/emulators and mods and cheats and piracy of non-physical games/DLC. The MIG only enables piracy/backups of physical games and can never do anything else. You can't even do save-based cheats with a MIG since the saves are stored on the console.

You criticize Nintendo's security for "letting any random cartridge run software" when actually the Lotus firmware is still one of the most secure parts of the Switch. RCM access is a holy grail exploit and even with it supercharging any Switch reverse engineering for the whole console's lifespan, the MIG is the only known thing that can pass a pirated game through the Lotus. And the MIG released really late into the Switch's life cycle, so it required a ton of engineering to make. It's nothing like the flashcarts from the NES-DS era at all. There is an analogous product to the MIG for the 3DS called the Sky3DS which similarly could only play pirated cartridge dumps and not be used for any custom code (though with the history of 3ds hacking, the Sky could be used for game-based entrypoints to full CFW like ninjhax or oothax).

I don't know what your thing about playing old carts is supposed to mean. If it's not clear, I'm pro homebrew and backups and believe that piracy (specifically, the defeat of DRM) is the only way to preserve software in any meaningful way. The MIG isn't a product I use or own, my Switch is a first-gen with the bootloader vuln. Those Pokémon carts also "held up" fine? The save file is vulnerable to battery death but the cartridge still "works" and there are methods to back up the save file or even perform a battery replacement without damaging the save. But that's got nothing to do with the MIG or the Switch.