The default model, where a single user is creating a single transaction, is not suitable for a large scale. Fees will be high, and doing solo transactions will be expensive. And then, sooner or later, you will have a choice: to join a group of people, and push a joined transaction on-chain, or to stop transacting, and wait for lower fees. And I don't want to wait, so I will just do some batching. The same with UTXO ownership: in the future, owning a single UTXO may be too expensive, so a lot of people will have their coins locked behind some kind of multisig, just to decrease their fees.
Another thing is to redirect the flood into other systems. For example, if Ordinals are confirmed on testnet, then people don't have to push it into the mainnet. Bitcoin would be much more congested, if it would be the only network. And it would need to be much more scalable, to handle all traffic from all subnetworks. And as long as we are not there yet, then test chains can decrease the load, because then, some projects can start on testnet, and stay on testnet, if mainnet fees will be too big for them.