SD cards don't have to be slow, but I doubt ultra-budget phones support UHS-2. The bigger problem is that Android places a bunch of limits on apps installed on cards, so many apps don't allow installation to the card.
The numbers for the past four years don't tell the whole story. Apple and Google are the only companies that introduced smaller phones that anybody would actually want. Google hasn't made one since 2020. Apple, since 2021. There's no indication the next iPhone SE will be as small as previous models.
>so many apps don't allow installation to the card
Why is that a bad thing? When you see how unreliable SD cards are you wouldn't want apps installed on them either. SD cards are good only as a temporary storage medium, similarly to how you use a USB thumb drive, to carry data for point A to point B, that's it. You don't want them for long term storage of Important data like you use SSDs or HDDs. It's not just about the medium speed, it's about reliability.
Yes, I know, premium SD cards with industrial reliability exists, but let's be real, no Average Joe ever buys them and instead just buy the cheap counterfit value SD cards that are on sale at Amazon or AliExpress or in best case at Walmart.
I did not argue that it's a bad thing many apps can't be installed to an SD card, only that an SD slot isn't a sufficient mitigation for inadequate built-in storage because of it.
I'll make that argument now though. I don't want an operating system telling me I can't do things because it might not be a good idea. Show a warning when something might not go well, but my risk tolerance regarding instability and data loss are mine to decide, not Google's and not some app developer's.
In an ideal world, probably yes. In a world where proprietary apps for Android/iOS are increasingly hard to live without, I'll settle for Android with root (though some apps try to make that difficult).
I guess there are 5/4=1.25 small phones per year. Fewer than I expected!