When there are 5 billion users, no individuals will be able to afford to transact on-chain - 5 billion people => 1 transaction per person per ~20 years - unless there is some highly scalable way of aggregating transactions.
There's a big if lying here. What makes you think billions of people will be incentivized to make transactions with bitcoin? Only a tiny fraction of them currently knows bitcoin, few of which do self-custody. I don't say they might never recognize benefits, but there are unbelievably many people who don't want to be responsible for their funds, and prefer leaving them to a third party, and countless of others who don't have the technical knowledge to hold some properly.
Lightning does not address this problem. Lightning improves transaction speed and volume, but it does not solve the root problem of redeeming one's coins on-chain.
What's "redeeming one's coins" anyway? Lightning isn't a common IOU. Sure, the coins only move locally, off-chain, but the system requires no trust to work, and it does reduce the median transaction cost.
My thought was that there needs to be some way of creating a multisig equal-value coinjoin/pool that allows a subset of the users in the pool to aggregate their signatures together and move their coins into another pool in a single transaction, while still retaining ownership of the coins.
Have you checked
CoinPool?