I believe most of these theories essentially suggest that the error correction required to produce a precise and reliable answer from more qubits, will grow faster than the computing power added by those qubits.

> no classical theory can explain quantum mechanics."

Not sure I can agree with 't Hooft on this. A GR-based theory with closed timelike curves can easily have particles travelling through time to interfere with themselves, and thus reproduce the quantum phenomena like the double-slit experiment. There's been some work on this:

https://www.reddit.com/r/Physics/comments/1gjekn/the_logic_o...