If they didn't use SD cards, the storage would be more reliable, but users would spend a lot more time fixing bricked boards. By allowing for removeable storage (in a format that can be plugged into any other computer natively) they solved that problem because I can just re image the card and get going again.

The philosophy of the RPi is that they won't really add features to it unless a bulk of the user base would use the feature. For example they were hesitant to even build the WiFi into it because users who wanted that could always get a USB chipset, and building it in adds BOM cost.

Because with GSM you'd also need a plan for it, I don't really predict they'd add that. Especially since you can get it in hat format already.