What do these micro SSD cards do to warrant 200?
Micro SDexpress cards are SSD-caliber high-speed, rapid access, sustained transfer storage devices in the size of a thumbnail, with heat management protocols in place so it doesn't cook the handheld. It operates over a PCIe bus and conforms to NVMe interface protocols.
This kind of memory is how the Switch will do instant loading and other "Velocity Architecture" features we've come to depend on in this gen of consoles and PCs.
Switch 2 needs SDexpress for its removable memory, just as it needs the onboard UFS to play games of this era. What it's packing will not be as fast as a PS5 or Series drive (we don't know about the onboard drive, but even the highest speed class of SDexpress cards are portable caliber, not desktop/console elite,) but it should do the job of supporting next-gen game features, and a new Switch without this rapid IO feature would languish behind the gen (and would drag down online experiences in cross-play.)
...Unfortunately, it's fucking expensive right now, and it could get better over time, but there's no promise that the situation will clear up any time soon. Switch 2 is the first real driving force for the SDExpress format in either full-size or micro (a few cameras offer it, but that's about it until now.) Eventually, SDexpress
should become the defacto SD card format, and
should level out in pricing as competition and volume of production finally kicks in now that SDexpress has a killer app. Until then (if ever?), it'll be a rough adoption timeline.
Should've been just use standar nvme SSD, but Nintendo don't want you to open the console
That's also a a fair-sized chunk of hardware to find room for in a portable; there's SSDs the size of standard SD cards, but space is bound to be a premium even for those, and then it'll still need heat sink. And it wouldn't be easily removable, as you said. The surgery needed to replace the drive in a Steam Deck is simple once you get it open, but that's still an extreme ask for a kid's device, and it still locks you into whatever you have onboarded at the time rather than offering a true removable storage option. The condensed cartridge-style Xbox Series add-on memory unit is nice and I wish that had caught on and developed into a more common format even outside of Xbox, but that's still a big card, and you probably can't get it much smaller at the time and be a fully self-contained plug-in.
Micro SDexpress is the right format in general. It's small, its familiar, it's backward-compatible (...ish, since Switch will weirdly barely even read standard cards,) and it is eventually going to probably be the ongoing future of SD cards. In time, it could be ubiquitous, and it could be in the price range of other high-end cards in likewise capacity offerings.
Unfortunately, the timing here sucks.
Why micro and not a full size SD card?
I believe the Micro SDexpress has caught up to and maybe surpassed the standard size SDexpress, which lingered in disinterest without a major hardware piece to make use of it (and other problems,) and now it's the preferred format for the SDexpress concept. Finally there's a platform which would have a hard time succeeding without its high access and sustained speed. There's not much benefit to it being bigger in this case; much of the bigger size is extra plastic, and the memory in the SDexpress format is stacked. Price generally doesn't make much difference, you're paying for the memory, not the shell, and ideally Switch 2 will normalize this format and help it go wide/get cheaper. So Switch 2 went with the format that
hopefully will be what works for the market.
Ironically though, because the cards are so small, people may balk at the prices for such a little doodad (that, and the low storage capacity so far, and the similarity/confusion of the standard Micro SD market.)