surely, the user will be taught to enable the hardware for video if they start seeing stutter. Or the browser can prompt the user to switch to "high-end graphics" if it detects prolonged video decoding.

If a website that has no obvious case for using the GPU, but is instead using it to fingerprint, then the user won't experience any slow downs from a software renderer (as it is usually done relatively quickly).

If a website needs the GPU for their videos/graphics, but also incidentally wants to fingerprint you, you're shit out of luck in that case. But this is no worse than what we have current day.