> Does anyone know of anywhere that managed to bootstrap good public transit after the fact?

Shanghai. Amsterdam up to a point - they never completely lost their transit, but it was in pretty bad shape.

> Anyway I wonder if congestion pricing could potentially be such a bootstrapping force, pushing enough people to use transit to start the virtuous cycle of increasing ridership and increasing quality.

It can help. You need improving transit and densification to happen together so they can reinforce each other, so you need coordination between transport policy and housing policy, I think that's the key.