Memory? You mean RAM or storage? We have had multi-user support on OS X from the inception, and that was on Macs with much less RAM and storage. Don't tell me an iPad with 6GB of RAM and 512GB storage cannot handle it. Multiple user login is a function of the OS, not the hardware.
I do agree it's not that Apple cannot do it, but it's not easy, especially since Apple focus on security and privacy on iDevices. Probably things like, how would the secure enclave handles the multiple user biometrics.
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Imo iPad OS is fine, and the fact that Apple spun it off with a different brand means they have different trajectory for it than regular iOS. We practically only at iPad OS version "1" (despite it being iOS13), so it still has a long way to go.