They already did that, L4S or no. "Fast lanes" usually come in the form of peering links or colocated cache servers, both of which involve actual new capacity.

They already did that, L4S or no. "Fast lanes" usually come in the form of peering links or colocated cache servers, both of which involve actual new capacity. Prioritizing individual flows of traffic over ordinary transit links based on monetary value is something IP is uniquely ill-suited to do.