Good point. Intuitively, if the new casual users would cover the costs of the old users' trips, then the old users would have to cover the costs of the new user

Good point. Intuitively, if the new casual users would cover the costs of the old users' trips, then the old users would have to cover the costs of the new users' trips. To further increase the average revenue per trip after reaching this balance, you'd continue to attract more signups by casual users. Whether this is feasible depends on the actual numbers, but at least it does work in other cities.