if billions of people used bitcoin on-chain, they would get a transaction every decade-ish. for an l2 to succeed at scaling it would consistently need at least a thousand off chain transactions per on-chain. since there's also going to be busy times, you probably want another 10 or 100x on top of that thousand.
but the real kicker is unilateral exit. imagine a billion people trying to unilateral exit at once. the only way to scale is to never touch utxo's, and since the working definition of an l2 is that you need unilateral exit, an l2 can never scale
