Gasoline is (something like) 20% of refinery outputs. I think a lot of that is coming out of upgraders, so there can be quite a decline in output before there's not enough gasoline refiners for there to be price competition.

https://www.eia.gov/dnav/pet/pet_pnp_refp2_dc_nus_mbbl_m.htm

(the upgrading part matters because they can shift their production around a lot more than if they are just fractioning the crude)