That sounds nice, but can you explain the economics behind it?

Users aren't paying a fixed fee, so we can't simply assume that more transactions = more fees.

If block size limits > demand for block space, then fees will drop close to 0 -- why would anyone pay when there is free space? Miners will include your transaction anyways.

Without fixed fees or a fee market based on full blocks, there is no basis for you to say that increased adoption alone will increase fee revenue for miners. You've suggested keeping a "buffer" between block size limit and observed usage, meaning no one would ever need to pay fees at all. With inflation dropping to nothing and fees dropping to nothing, you must expect miners to secure the network out of charity!