Again, even if a single user in the pool theoretically has the power to unilaterally withdraw their funds, if it is cost-prohibitive then it might as well not exist as an option.
There's a cost for everything. Perhaps there could be a mechanism, wherein the user who closes the channel / pool pays most, but there will always be a cost. If you increased the block size, the fee rate would surely drop, but the cost would later be translated in decentralization.
If I'm keeping my life savings of 100,000 sats in a CoinPool, and the transaction cost is 50,000 sats, then the option to withdraw my funds unilaterally basically doesn't exist.
You talk with numbers, but there's no indication that you will pay 50,000 sats for an on-chain transaction.
Regardless of what the cost is in sats or dollars, the fact is that 500,000 transactions per day / 5 billion people = 1 transaction per person per 10,000 days.
Even if Bitcoin were one one-hundredth the size, it would still be 1 transaction per person per 100 days, which is unworkable.
If everyone were using LN - assuming channel factories become a thing - then 99+% of people would not be able to afford to receive any of their sats onchain.
Their theoretical ability to receive/redeem their LNBTC onchain would be meaningless, because the transaction cost would be a significant portion of their total wealth.