Superconducting does seem much easier under a few hundred GPa's of pressure - but that's less convenient to maintain than liquid helium cooling.

Disclaimer - all I know about superconductors, I know from high school physics, and I left high school some 35 years ago so I know the State of the Art is waaaay over there somewhere now, and here I am still playing with my mercury cuprate stuff.

Anyway.

You have a car. It's similar to my car. It has a 200bhp engine, weighs about two tonnes, and tops out at about 100mph. How would you make that a 200mph car?

Well, you'd need more energy, for a start, but E=1/2mv^2 turns into sqrt(2E/m) right, so you need four times as much power for twice the speed. This is okay. You're not getting 800bhp out of the engine you have now but it's doable. You can buy cars with 800bhp engines, these days maybe you'd be looking at some electric motor.

But you're still not doing 200mph because the drag increases as the square of the speed too, so you'd actually need 1600bhp to get to 200mph, which is still doable but opens up even more problems because now everything needs to be heavier to cope with the power.

So all else being the same you're actually onto about 2400bhp or so before you crack 200mph.

Which you achieve just as you either run out of road, or more likely petrol, at £1.50 a litre, so you're not taking too many attempts at it.

Anyway, the tl;dr - it's not just one thing that's stopping you getting that transition point higher, it's a bunch of stuff that interacts in weird ways.