The more recent choice would have been ExpressCard, but that standard was never updated beyond PCIe 2.0 x1 and USB 5Gbit/s. But the bigger problem with both ExpressCard and PCMCIA is that the cards are long—those form factors date back to an era where expansion cards needed a lot more PCB space than they do now. ExpressCard 34 is 70/75mm long, compared to a typical client WiFi card that's M.2 22x30mm. It's quite impractical to have ExpressCard slots on both sides of the machine.

I don't think it would have worked well to try to make a hot-swappable externally-facing card form factor derived from M.2. Likewise, cutting EDSFF E1.S down to a third of the length wouldn't leave any provision for USB or DisplayPort signals. USB-C is clearly the best available connector choice among current standards.