L2s will break composability -- which is a pretty large cost. They're neither composable with ETH, nor meant to hold a lot of composing apps within them.
Composability is a *huge* amount of the power of DeFi.
Devs: many ETH devs won't, some will. But there are also other devs
Also, I think it's important to keep some higher level context when you describe protocols.
You say "no one is fucking using Serum", I say "it's the highest volume DEX that isn't paying to inflate numbers".
Or, possibly, "no one is using DeFi, compared to CeFi".