It wouldn’t be useful to them. Among the modifications they made to pad 39A, SpaceX built their horizontal assembly building right where the crawlerway approaches the pad.

Probably not. Falcons are assembled and transported horizontally, thus don't need a mobile launch platform. Starship will probably be too big for it, and will probably need its own solution for assembly given its unusual launch and landing profile.

So far the SpaceX Starship solution seems to be commercially available cranes, commercially available "roll lifts" (flat beds with a lot of independently controlled wheels [1]), and simple launch sites that don't move around on land.

I have to imagine that SpaceX's solution is orders of magnitude cheaper than maintaining this giant custom vehicle NASA made.

[1] https://i.redd.it/wo6buoppcov31.png