When what you call spam becomes uneconomically, that will mean fees are higher than they are currently. Which means less people will be able to afford onchain transactions.
Fees go exponentially down over time as issuance goes down over time. The first fees were like 1btc to move anything. The reality is 95% of people will never control a utxo. They will have shared custody of a utxo or straight non custodial btc.
I do agree that as fees go up, less and less people will be able to self-custody. Over 99% won't eventually.