Not just that – it complicates cell planning a lot.

As far as I know, UMTS and LTE behave very differently under load, and at least in the early versions of UMTS there was the phenomenon of "cell breathing", where the serviceable cell radius varies with network load (due to the CDMA modulation/multiple access scheme used).

Having to provide denser cells in sparsely populated areas just to avoid dropped calls during the odd traffic spike sounds like a real pain.

In my old house, on UMTS, I would have perfect signal except for 6am-4pm when the local school was there. It was fine in the summer. The local school soaked up the entire “cell” and my house was a dead spot.

It's more about these devices that aren't so easy to replace like car modems. If 3G won't be it then we need to settle on something else that can be promised to be there in the foreseeable future for the longevity of IoT/cars/whatever else.