The problem is that there isn't much more we can do to bring down the cost in tunnelling - there has been incentive for many, many decades to get the cost down

The problem is that there isn't much more we can do to bring down the cost in tunnelling - there has been incentive for many, many decades to get the cost down to the minimum possible, and that's basically what happened. New materials and things might make small improvements going forward, but it's an extremely mature industry. That's why the Boring Company hasn't made any progress on this grand vision - they seemed to set out by assuming there were easy wins to be had (seemingly not consulting any actual experts in the field), and then there just... weren't... Basically the only thing they have been able to do is to make smaller tunnels that lack any kind of emergency egress - which is kind of important in real mass transit systems where people would rather be able to get out if there was a fire.