That's an incredible inelegant solution though, and one I don't think Apple would ever implement.
I think they'd rather release another 6K 60Hz display with more refined mini-LED technology than have users plug in with two connectors.
But never say never... 6K 120Hz is technically possible with the highest bandwidth DisplayPort 2.0 as well, as it's just about 74Gbps at 8-bit pixel colour depth. DisplayPort 2.0 – which is based on Thunderbolt 3 – supports up to 80Gbps. Maybe Apple will let users choose. Maybe they'll compromise and go for 90Hz, which is still a substantial improvement on 60Hz, while requiring almost 20Gbps less bandwidth at 8-bit colour depth, while still allowing for 10-bit colour and remaining DP2.0-compatible.
Or maybe this is all just a pipe dream, and the bandwidth limitations are just too much for Apple to consider a 5K or 6K >60Hz display for the time being, and Ross Young is wrong about this one. I hope he's not, as I really want that 5K mini -LED ProMotion display, but he could be.