Those things are less static than you seem to believe. Eg recurring prescriptions; at least once a year my insurance gets mad about my scripts and requires a pre authorization that occasionally contains a stipulation that I try their “preferred” (read: cheaper for them) alternatives.

My planning is now wrong. Likewise I can only do FSA for planned procedures if I know close to a year ahead of time, which isn’t super likely. What portion of procedures are serious enough that insurance won’t call them elective, but also tame enough that you can put it off for a year for your FSA?

I don’t think I’ve ever had my FSA be completely correct. I’m always either over-stocked and losing money, or under-stocked and paying with post-tax dollars when I don’t have to.

Again, the FSA is for known expenditures, not unknown or variable. It's not perfect and I'm not really even defending it; I think an HSA is generally a better idea but the FSA does have certain specific advantages.