A concept like this was proposed and evaluated for inclusion in Blossom. (As I recall, it was originally my idea.) Gordon Mohr’s objections to it, posted on that github thread, seemed pretty persuasive to me. Basically, constraining blockspace probably wouldn’t have the desired effect, currently, in the under-loaded state. I don’t remember what we thought would happen if we hit an over-loaded state. But, the intention of Electric Coin Co is to make Zcash scalable before we hit the overloaded state. Gordon argued on that ticket that imposing what he calls “demurrage” on t-addresses would be more effective. I think “demurrage” is basically the same as fees, as proposed in PRINCE — Privacy Incentives for ZEC and in proposal to charge higher fees for spending old UTXOs and Notes. #3693.