There are speed and capacity bottlenecks in the SD form factor, and then the investment of putting newer tech in it can be considered regressive in the face of CFExpress picking up momentum. But the above GoPro example is a legit use case to stick with (micro)SD as long as possible. I'd say SD is not at its prime, it was probably when practically all non-Apple smartphones had a microSD slot, but now phones or other small form factor electronics have a tendency to use eMMC instead for better or worse. Devices that clearly need portability or disposable media swap like action cams or dash cams, will probably use it forever. Then some "serious" cameras especially SONY's, they have strategic reasons to stick with SD, not as much a technical reason.
It would have made a lot more sense if the SD slot stayed during the 2016-2020 touch bar MBP era, and then removed during the move with Apple Silicon, but it turned out the other way round. Apple's decision to put the slot back I'd venture to guess has a large part to do with PR than actual demand. It is very unlikely that there are more MacBook users needing the slot than 8 years ago.
What's interesting is the HDMI port, or its standard even, got more relevant in computing space though; during the Thunderbolt 1&2 era it was clear(er) that the computing industry would rather stick with DisplayPort, at least with hardware that involves Intel. It looks like there was a huge consumer push in TV sets and consoles that forced the port to be supported as well as possible on PC / Macs for now. What I mean is that the HDMI (2.1) port on Macs is more important than it ever was, especially right now it is a more capable video output than through DP alt-mode on the USB / TB route. So the MacBook Pro has to have a full sized HDMI port anyway, given its thickness that means adding an SD slot to the chassis is not much of a design-cost so Apple probably thought well there you have it. Conversely, if the SD slot was deemed important, they would have added that on the MacBook Airs as well. But one can argue it may be a case of product segmentation to only have it on the MBPs.
