Thunderbolt would've been sick. For many other reasons than PCIe expansion, as 40Gbps might not cut it as you said.
remember that most people on this sub would, and most people on this sub probably know how to install RAM/graphics/IO cards onto thier PC, and even if they haven't would be perfectly able to.
the general public... not so much. in fact sony's solution is likely to induce confusion.
Its probably part of the same reason the switch does NOT support bluetooth headphones, as it can't connect to 8 joycons and do audio at the same time, even if the other technical issues with bluetooth didn't prevent it.