But that's ATC's job to worry about.

This is the correct technical answer. I'm not knocking ATC, and another post clarified that they know to protect the "overlapping" vertical space when pressure is below standard, but humans make mistakes. No one is citing any mid-airs that happened around 18,000 MSL because of conflicts, so it doesn't appear to be a significant risk.

That said, it's interesting that there isn't something like a 1,000 or 2,000 ft buffer like "VFR tops out at 18,000 MSL, and it's Class A above that, but ATC doesn't route anyone below FL200." But like I said, this doesn't seem to be a problem that needs to be fixed.