you have already made the presumption that on-chain spam will be constant, and that fees will be constantly high because of that
Yes. Because in the long-term scenario, if you have to share the full chain history from 2009, up to today, instead of sharing a cryptographic proof, that all of such history is correct, then you discourage many people from joining the network in the first place, and from making new full nodes. Which means, that if the chain will grow over time, then if our ability to synchronize the chain wouldn't grow accordingly, then it would take more and more time, to sync the full chain. And that can affect decentralization, because that approach doesn't scale.
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.
On-chain fees should be constantly high in the future, because otherwise, miners wouldn't have enough block reward to have enough incentive to keep mining the chain. As Paul Sztorc wrote in his Sidechain Vision,
"Fee Revenues must rise". In practice, it doesn't mean, that the on-chain fee should be paid by a single user. You can have thousands, millions, or even more users, making their transactions on second layers, having all of that batched, and finally pushed on-chain, as a single transaction with high fee rate. If you want to scale seriously, you won't onboard everyone on-chain. Which means, that you will have a lot of users, making transactions only inside LN, only inside sidechains, or only inside other kinds of subnetworks, and all of that traffic should be batched, and confirmed on-chain. In this way, you can have thousands of users, paying a bunch of millisatoshis each, and have a single on-chain transaction, paying a bunch of satoshis, signed by all of them, and covering all of their needs in just a few kilobytes per batched transaction.
Check the current state of the mempool.
The current state of the mempool can show you, that a lot of traffic moved outside Bitcoin, which is why it could be bad in the long-term. Many people stopped using Bitcoin, just because of scaling issues. And if they won't be solved, then you will have only whales transacting on-chain, and all users will use centralized solutions, provided by those whales. And that's why decentralized second layers are needed, to not lock users into exchange-like or bank-like systems, where they have to trust, instead of verifying.