That said, having worked on account/identity systems at another FAANG, I think that the commenters saying that Apple is holding this back purely to sell more iPads are underestimating the complexity of this feature.

This is not a feature that you just bolt on to the top. It will require a significant ground up rewrite of iOS' fundamentals if you want to support account switching without a full shut down of the device (and even with that, there are complications with shared storage).

There are likely tons of singletons across the iOS codebase for the "current account", and switching between users will easily lead to bugs where the new account shares/accesses state from the previous account.....and these "violations" are much harder to detect via static analysis than you might naively imagine.

UPDATE: I wasn't aware that Apple already supported a bunch of this via MDM. My only point was that if they didn't already build this into the foundational layer of the OS, then this is a very difficult feature to add later. If they already have this, then I don't have any defense left for them.