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.
Block size was never and will never be a sufficient scaling mechanism, at least not on its own.
If we talk about signature aggregation as a scaling mechanism, have a look at MuSig2 - it sounds pretty promising to me. Combining signatures off-chain instead of on-chain saves fees and blockchain space, while not requiring channel opening and closing transactions.
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It's good that a MuSig2 BIP is finally about to roll though. It's about time that something like this happens and has profound implications in both scaling with off-chain threshold transactions or with aggregated transactions.