The cynic in me says that Apple couldn't figure out how to get 4 tb4 ports on an M1 so threw some single use ports in as a stop gap. I just feel like if they had really sat down and designed the computer around adding these ports back, the latest version of HDMI would have been used.
As for the SD cards, I have mirrorless and dslrs that use sd cards, and don't really care about sd in the machine or not. I think many people who are clamoring for an sdcard want to use it as quasi permanent extra storage. /shrug
I tend to agree with you that the HDMI port is actually a 'stop gap' - TB4 mandates 32Gbit - so every port requires the equivalent of a 4x PCIe3 link, (this is what the previous models used to drive two ports).
So 4 TB4 ports would mean 16 PCIe3 lanes (I can't find if the M1 derivatives are v3 or v4 - if they're v4 I believe they'd require half the lanes per port as the lanes are double the speed), which maybe is more than they wanted to allocate.
The only way I can see this makes sense in anyway, that isn't just "Apple cheaped out on the bandwidth" is that they were caught off guard by Intel's TB4 (which sets the minimum bandwidth requirement) announcement last January, just 6 months before they announced they were transitioning off Intel. They no doubt had the CPU designs at least somewhat done already, so maybe they'd been planning for TB3 ports - but even in that scenario, it's a cop out if their brand spanking new CPU designed in 2021 can't support more external I/O than it's predecessor from half a decade earlier.
I've seen people saying the SD card is "a great option for a storage upgrade".. I guess it's an option but I wouldn't call it great, given the speed difference compared to a modern NVMe (either the built in drive or an external one).
As for the SD cards, I have mirrorless and dslrs that use sd cards, and don't really care about sd in the machine or not. I think many people who are clamoring for an sdcard want to use it as quasi permanent extra storage. /shrug