I seriously question "massively profitable", especially if you do it as a private company. You'd have to buy the right-of-way, and then pay taxes on it every year. That's some pretty expensive real estate. Then, the maintenance requirements on high-speed rail are pretty stringent. That costs, too. It's not just the cost of the train crews and the electricity (or fuel) that you have to think about.
The Acela is already above 120mph where it can. The biggest problem area is actually in Connecticut, and half of that issue is Metro North's fault, not Amtrak. (The other half is that the line is legitimately too curvy to support 120mph speed in large places, but Metro North is why it struggles to hit 70mph at times).
A thick log or dead deer across the tracks of a high speed train could lead to catastrophe.
I once rode the Eurostar from London to Paris and as I recall all the high speed sections were enclosed in very high, barbed wire fence. So some security is definitely needed. Collision energy goes up with the square of velocity.
And I'm talking above 120mph. None of this 70mph "high speed" slow stuff.