But some of the costs of satisfying external stakeholders, like buying land; getting approval from regulators, politicians and the local community; and so on would be independent of tunnel size.
IMHO the main reason not to install a conventional metro line is Musk wants to demo something going 150mph, and I've never seen a metro line going that fast.
It may seem counterintuitive, but higher speeds mean less throughput. Most metro systems are concentrated on trying to move high numbers of people, so bumping train max speeds to 150mph just makes no sense. The sweet spot for speed appears to be around 50-70mph, which most metro trains will do when they're cruising between stations.
But anyways, anyone who is complaining that local mass transit ought to be going 150mph is someone who hasn't looked at why local mass transit can't go 150mph or is someone who doesn't want the "mass" in "mass transit".