I think the only way SD would be able to get a new airport anytime in the (relatively) near future would be to build a modal airport that would consist of three parts.
1. The Welcome
Parking, ticketing, baggage, rental car operations, and security checkpoints built somewhere here in-town...maybe even at Lindbergh since much of the infrastructure is already there.
2. The Link
Once folks get through security, they hop onto HSR trains which take them to the the planes.
3. Da plane! Da plane!
Somewhere in the middle of nowhere is the rest of the aiport - the runways, airplanes, control tower, and gates.
I expect that the most expensive and potentially infeasible aspect would be the HSR line, but at least the thing is not floating in the middle of the freaking ocean!