For short trips. Most plane trips are at a distance where there isn't a viable alternative. Sure there are some nice trains that can handle the more common trips, but airplanes really shine for much longer trips.
For rail? You're looking at an order of magnitude more for planning and construction, all new infrastructure, all new rail stock. It will take a much larger passenger base to justify the expense and effort.
Airplanes don't just shine for longer trips. There have been some interesting developments with commuter airplanes that are either all electric or hybrid with a 200-plus-mile range. If they are as energy efficient as they promise to be, we could see a resurgence in municipal airports for transportation between regional centers where rail is either unavailable or excessively expensive.
Some of those are amenable to high-speed variants, some less so. And once you've established transport corridors, much of the hard part (rights-of-way acquisition) is accomplished, so that adding a high-speed route becomes far more tractable.
For the most part, actual construction costs are fairly modest, modulo bridges and tunnels, both of which can of course be quite expensive.
I think increasing the viability of travel between smaller airports is a good thing for those times when you don't want to spend a lot of time traveling or waiting for a transit vehicle. If the equivalent of micromobility air travel comes to fruition, you'll see two- or three-person vehicles that get up and go when someone wants a ride. Another benefit could be the airport-to-airport delivery of packages with different-sized drone craft and autonomous navigation.
#IFoundMyFlyingCar