Boring Co is using a second-hand sewer tunnel boring machine. This isn't the future of tunneling, it's quite literally the distant past.

Modern mass transit uses bigger tunnels because bigger tunnels means more throughput. It's generally cheaper to dig two (parallel) big tunnels than a dozen smaller ones, since the fixed costs are such a large part of the expense.

This is the part that Elon doesn't understand: it's not all about opex. Capex matters to, and for infrastructure projects, capex dominates opex. (For example, if he hadn't wasted a billion on the dreadnought, i.e., capex, Tesla wouldn't be in the doldrums right now unable to fund its basic operating activities, i.e., opex.)