People will just start commuting to further away places like from barcelona to london: https://www.theguardian.com/cities/2015/aug/11/commuting-fro...
You can argue for an economical benefit, but it's very hard to argue that they are larger than the costs.
Elon's tunnels don't solve anything. His cars are so large that the amount of people a tunnel can move is small. We can add more tunnels, but it isn't cheap, and geometry means we can't add enough - there is only so much space below us and you need to keep tunnels apart.
> there is only so much space below us
Is this actually a practical limitation? You could go a mile deep, and pack the tunnels way more densely than roads on the surface (since we don't need to put buildings in between the roads).
> But tunnels do remove many of the arguments against cars: pollution (both CO2 and tire abrasion) can be easily filtered
I have never seen this done. Questions: How? And if it's so easy, why don't we do it?
Viewed from a different perspective: we have so little capacity that most journeys are never taken.
I agree that in general mass transit is much better than cars. But tunnels do remove many of the arguments against cars: pollution (both CO2 and tire abrasion) can be easily filtered, noise is blocked by solid rock, tunnels can be crossed without knowing they are there, they don't interfere with pedestrians, don't reduce building density, don't create artificial barriers. Cars are still incredibly dangerous, but Elon's sledges would solve even that. Cars still take up a lot or room both in transit and when parked, but most other disadvantages are reduced or eliminated by putting them in tunnels.