People will still mint, trade, send, receive their dick pics and fart sounds on-chain on the Bitcoin blockchain simply because "they can".
Of course. But then,
people willing to use Bitcoin as a P2P money will simply move to other subnetworks, while leaving on-chain spam, where it currently is. And then, you will have a choice: use the current client, and process all of that money-unrelated spam, or upgrade your client, and focus on monetary transactions.
--Snip--
Let me stop you there because you have already made the presumption that on-chain spam will be constant, and that fees will be constantly high because of that. I believe that we should also consider the fundamental economics of the fee market. If the price
forof a good or a service is high enough, then there would be decreasing demand for that good or service.
Fees will NEVER be constantly high because users will be priced out at some point, and many of them will wait for a lower, more favorable price.
There will be some shitcoinery in Bitcoin, but it will never be as massive as those shitcoin chains such as Ethereum and Solana. Check the current state of the mempool.