How do you charge people to stop at the terminal without severely crippling an already saturated people drop off / pick up pipeline?
What would probably work better is a toll ticket just to enter the airport complex via the main gate. Airports (almost always) have front gates to lock the entire compound if necessary, add a toll booth.
The visit toll could be collected electronically, via transponder or a bill mailed to the car's registered owner, to require no slowing or stopping of traffic.
They could also allow free or cheaper visits at the far end of the SFO 'airtram' (where the existing rental car center is located).... the differential could even vary with congestion.
If they want to throw a bone to occasional, casual flyers and friends on pickup/dropoff duty, let each license plate have 1 free visit each N days. (Though, the logic and fairness of charging every visit equally based on its congestion impact is strong.)
There are better ways to meet any legitimate concerns of SFO than to force new services into the old rental-agency cookie-mold.
SFO's "legitimate concerns" are that they can control activity on their property. If they want to charge rental car companies extra, that's their right. This isn't society-wide regulation, it's a business entity (one that happens to be owned by a government) deciding what to do with their own property.
A regular business could not issue the tickets SFO is issuing. A regular business would also be subject to additional antitrust and fair-business-practice constraints.
So if you're suggesting the airport be privatized, and then offering services like any other "business entity… deciding what to do with its own property", I think that'd be a great plan.
But SFO is a public entity, using state power to enforce extra monopoly privileges. It doesn't get to just say "because we want to do it this way with 'our' property", even if it's silly. It's the city of San Francisco's property, and their actions should be serving the public interest.
An equitable regime of charging based on actual visit impact achieves that. On the other hand, enforcing the old formulas out of tradition is lazy and destructive - and suspiciously protective of SFO's longtime partners among the incumbent rental agencies.
What would probably work better is a toll ticket just to enter the airport complex via the main gate. Airports (almost always) have front gates to lock the entire compound if necessary, add a toll booth.