Not that I think it was a good reason for ditching them whatsoever, but one of the main reasons given has a tiny bit of merit:
Allegedly, a lot of users often cheaped out on SD cards and then used them as adopted system storage where they moved all their apps to. This had the effect of completely tanking phone performance and then they would often get corrupted after a bit of use taking user data with them. This then generated a load of negative reputation about the devices being slow and unreliable, when the problem was the choice of SD card, and generated a load of wasted money in supporting these users when that happened (think unnecessary RMAs, etc).
Personally I think they should just have restricted it to A1+ SD cards, and sucked up the people complaining about their bargain bin Scrandrisk SD not working. But I guess they saw an opportunity to have their cake and eat it by just removing it and charging a premium for larger storage skus.
Question is rather: why does Android not allow any distinction between Internal and external/removable?
My downloaded media files belong on the SD card, but APKs, sqlite DBs and temp files don't belong there. But de facto, it's just used as an extension for internal storage. That's just stupid.
I recently copied all my files from my SD-Card ("formatted as Internal Storage") to a safe location, then copied those files to the "Internal storage" part of the rom that opens up after the SD-Card was gone. It worked. on Android 10. Maybe that change was later. Android storage looks weird from the outside anyway. It was as if only the publically visible files Apps want you to see got on the SD-Card, and then each app has some hidden folder (that apps like termux or apps with custom-built file pickers sometimes kind of let you half-guess the structure of (my head: no details) and) that appeared to have been on the built-in rom all along. That copy operation now lets me remove my SD Card w/o moving all my Images and less than half of my apps files. </irrelevant-context>
It's not the users saying it, it's the OEMs. Back when this was a new discussion, I at least remember Google saying this as justification for why it stopped including them in their devices after the nexus 4
I don't think it's a lie, it's objectively true that shit SD cards would have that effect on performance if used as adopted storage. But I do agree with you that it was a convenient excuse, as I wrote at the end of my original comment.