I wouldn't underestimate how verbose JSON over HTTPS is, compared to a binary protocol that maybe only sends a single symmetrically encrypted UDP packet per sen

I wouldn't underestimate how verbose JSON over HTTPS is, compared to a binary protocol that maybe only sends a single symmetrically encrypted UDP packet per sensor status report. Some IoT devices run on CR2032 Lithium cells, and even a single order of magnitude still matters there. Network data usage can be a consideration too. Some IoT devices use satellites as a backhaul; you definitely don't want to (and in fact can't) run HTTP over something like Iridium SBD, for example.