I don't think he's claimed that the boundary was discontinuous, you've implied that.
However, there's no denying that QM and classical mechanics are two fairly different sets of equations.
I haven't come across a QM book so far that explicitly (i.e. with an example) demonstrates how one morphs into the other as a variable, say size, changes.
Special relativity is way better in that regard, when speed goes to zero, the equations converge to their classical analogues.
Ehrenfest's theorem would be an example where Newton's 2nd law is recovered as the wavefunction's spatial extent is reduced on the scale of the system being studied. More generally, you won't get an explanation that shows a morphing based on size because size is not the limiting factor. It's how isolated your system can be from the outside world. This is obviously much much harder as your system gets bigger. Decoherence theory (https://en.wikipedia.org/wiki/Quantum_decoherence) recovers classical probabilities as a system interacts with its environment.
I too find it unlikely that there is such a discontinuity. Quite a large number of physicists (and quantum computing people) seem to speak as if there is one.
> Finally, decoherence has seen plenty of study
I get the impression that decoherence is treated as merely a problem for technicians working on experimental apparatus, or an engineering issue to be dealt with on the road to quantum computation. Are there any results on fundamental limits to the degree of entanglement in a system? We've been trying to get that damn cat into superposition for ~85 years now, with no luck.
We've been setting records for bigger and bigger superpositions every few years - apparently we're up to 2000 atoms now. There's no reason to suspect there's a fundamental limit, any more than there's a fundamental problem that prevents nuclear fusion from working. That doesn't mean the engineering isn't devilishly hard, maybe even impractical (if it gets literally exponentially harder to maintain more and more particles in superposition), but it's not a fundamental physics issue.
The reason they seem to speak that way is because they are being a little sloppy for the sake of brevity. The physicists all understand that there is a blurry boundary.
There is an amusing instance where there was a similar confusion though: the soft diffraction limit was mistaken by many for a hard one. So super-resulution came as a bit of a shock.
My feelings exactly, I really don't understand why such a relevant comment is being downvoted.
I expect a theory predicting otherwise to bear the burden of demarcating exactly where the boundary between "macroscopic" and "weird not-macroscopic" lies (waving your hands and mumbling "thermodynamics" does not count). I expect that such a theory's proponents should be absolutely burning with interest in probing, studying and understanding what goes on at this peculiarly discontinuous boundary in Nature.
Those two things haven't happened, which is kind of sad.
Quantum Mechanics predicts the results of experiments better than anything else. If "predicts experimental results" is the only thing you care about, then Bohr was right, QM is complete, and there's nothing more to be written in that book.