So how are ENS names expected to be adopted in a world where no one actually transacts on L1?

Users definitely need to be able to purchase ENS names on L2s, but this seems technically challenging given the current design

building full stack onchain apps. making code go brrr. Building /puzzlebets --
doing stuff @ eth.co

I would be worried that the extremely low transaction fees would increase spam names, which are already still bad on L1.

Invest in an L1 name. The L1 name can still point at ALL your L2 assets.