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.
I don't see why the fee rate would be high. Payment channel settlement transactions just pay nominal fees, no matter whether they result from a handful of txs, or thousand of txs. This is not a case of packaging a bunch of txs into a batch that preserves all their fees, but of one final settlement tx obsoleting all earlier ones.