Earlier quoted context omitted.
Also, the passenger miles per unit of energy drops geometrically, as speed increase linearly. Most of the loss is aerodynamic, so you either need a hyperloop tunnel or wings to take you into the literal stratosphere, to avoid high fuel burn at high speeds, so even if you could run the train at 400 km/hr along the whole track, it would be unlikely that any operator would do so, on a recurring basis.
I don't think cost is too much of a concern for ICEs. They are already a higher-priced option that competes with plane travel for many destinations. You can take RE trains at virtually no cost between any two cities you can dream of, yet people en-masse still happily fork over a hundred bucks to save 3-4 hours with an ICE train.
I'm sure in some cases, an RE is a bit more competitive with an ICE, but in plenty of others you're asking for a pretty unpleasant time. (And this is from someone with a Deutschland card, making RE trains "free".)