Thing is, if you build an aeroplane, it can land at any airport. Should demographic changes result in lots of people wanting to suddenly fly from A to C, rather than A to B, it's very easy to accommodate that change. However, a fixed railway, and a VERY expensive ($/mile) one at that could be left high and dry if the demand changes or just moves away.
The UK is proof of this, as the rapid boom in railway expansion in the mid 1800's (the so called Railway Mania https://en.wikipedia.org/wiki/Railway_Mania ) led to massive building of intrastruture, that was later abandoned when the demand disappeared.
Increasingly, thanks to fast networking (optical fibre internet) people no longer actually need to physically travel between places at high speed, and i can' t see that changing?
I would have agreed with you wholeheartedly and thought it was so obvious it didn't need to be said, until I tried the high speed train around here.
If I fly from Boston to NYC, I get to the airport an hour early. I deal with security and all that BS and I have the problem of not being able to bring liquids (a hassle for toiletries). I land an hour after takeoff, but I am still an hour outside of Manhattan if traffic is light. If it's not, add 1.5 hours. Even if I take the subway, it's still an hour. Then I have to do the reverse on the return journey - it's a huge hassle. And it's also a pain in the ass if I want to change my flight time.
With the train, I show up and literally walk right on the train. I can get there 2 minutes before it departs. It departs on time right to the minute, and it arrives exactly 3 hours and 26 minutes later in 34th street in Manhattan regardless of traffic. I grab my bag and walk off the train and I'm in downtown NYC. I can bring whatever I want on the train - no liquids ban. I have high speed internet, I can spread out in a big chair with a table. I can get a sandwich, have a beer, use my cell phone, whatever. I used to think Amtrak were crazy to charge more than I can get a round trip airline ticket for, but now I would always choose the train over plane any day. That's why it has taken 70% market share from the airlines.
Sure, it's less flexible in terms of cost of building and changing the route - but Boston and New York City aren't going anywhere, and from the customer point of view, the cost to build it are immaterial - all that matters is cost, convenience, time and experience. Amtrak is expanding their network and gobbling up additional market share in the North East corridor (Boston/New York City/Washington/Philadelphia). It won't work in many places, but in the places it does work, I think the up-front cost could be worth it.
Another thing about hyperloop - if it ever comes to pass - is it would also be useful (maybe even start off being used) for freight. There are a HUGE number of freight shipments into NYC. A 30-minute courier service between Boston-NYC would be valuable. I've often thought that some sort of constantly moving tunnel-based high-speed conveyor belt or small-diameter mini-subway system would be great for big population centers. It would reduce traffic and congestion, allow cheaper and faster ingress and egress of freight and be reliable and fast. And without carrying people, the regulatory and maintenance requirements would have to be a lot looser.