Not every payload (or even batch of payloads) needs super-heavy launch capacity, but that seems to be what SpaceX is doing.
https://www.space.com/38323-spacex-phasing-out-rockets-for-mars-bfr-spaceship.html
SpaceX plans to eventually phase out the big rocket, which has yet to fly, as well as the Falcon 9 booster and Dragon capsule
Why? The cost of launch is going to go right back up once SpaceX gets out of the medium lift market.