But this is not 12ax7. And nutube is a cold cathode tube. There have been attempts to use low-power tubes called nuvistors before. Those are actually more of a tube than nutube, and they failed to get any traction. It's a notoriously fickle market, and tubes are still manufactured in China, Russia, and even in the US, so it's arguable if they need to be "replaced" by some newfangled thing.

Indeed, last I checked, a 12AX7 is like 5 bucks in quantity (and I'm no master at low cost sourcing), so it's barely even an economic argument. The guitar amp can't be made much smaller overall because it still needs a speaker.