Sure, with increased efficiency and better energy economics, that's not a problem.
Sure tons of cost for R&D but just because the tunnels are smaller doesn't mean they could only be used for cars in the long term.
You can see one in the CG demo, in the first few seconds of the video, on the right:
So about 250 - 700 people are moved in 3 minutes, that's 80-230 per minute. If sleds follow each other by let's say 2-3 seconds, and each car holds 1-8 people, then that's 20-240 people per minute. With the added benefit better granularity, and you don't have to switch lines, metros, you don't even have to leave your seat.
Also modern subway tunnels are typically constructed with a larger diameter than what the Boring company supports.
That said, I don't think American cities could run subway cars as small as in the London Underground even if they wanted to because of the Americans with Disabilities Act (ADA).
Also, it might be difficult to make cars that small that pass muster under Federal safety regulations. AFAIU, Federal regulations basically require subway cars to be built like tanks.
EDIT: >24 feet just seems too wide for the old, deep tunnels in the U.S., but I can't find any references that aren't from books at the turn of the 20th century, which have very inconsistent numbers. I said >24 because that seem to be the width of old cut-and-cover tunnels and newer deep tunnels. I'm going to go out on a limb--based on random sources--and guess that the really old tunnels still in use might be closer to 16 feet in diameter. Feel free to correct me.