A single-tunnel Boring Loop isn't competing against the 10th busiest subway, though. Even if Boring Loop were competing against New York City Subway, at $2.5B/mi you could build something like 50 parallel lanes at the same cost, and 2350m/50 is only ~50m. Space them out a bit so it's actually driveable and you're probably still ahead with Boring Loop, even assuming the trains are running full (which is not default).
Now, if you did invest enough for 50 parallel lanes you clearly wouldn't want to do 50 parallel lanes; they pack awkwardly, don't make sense to centralize that much if you design doesn't force you to, and you go from having a regulatory advantage of being able to build under city roads to a regulatory nightmare of asking to build 50 parallel lanes underground. It makes much more sense to build point-to-point connections out like a road network, and bring in more traditional mass transit where you have particularly busy connections that can't be handled with a reasonable number of lanes.