AVX-512's richness to x86 is like what C++'s is to C. Linus makes a summary assessment for how he can leverage these technologies to his advantage and if the cost of learning the technology and all its intricacies outweighs the perceived advantage: that technology is garbage. This reaction from Linus appears to fit his conservative pattern. I think where Linus gets things wrong stems from his facts rather than his philosophy.

AVX-512's fantastic breadth is born out of an actual need to free compilers from constraints imposed by programs in virtually every mainstream language. All of these describe programs for an academic-machine rooted in a scalar instruction model. Without any further performance from increasing cycles over time the target has to become instructions-per-cycle and even operations-per-instruction. The limitations on ILP and the expense of powering circuitry to achieve it has been well studied for the past two decades. The failure to realize it is evident in the failure of Netburst. Linus believes that the frontend of CPU's have a lot more to give; perhaps best exhibited with his refutation of CMOV (https://yarchive.net/comp/linux/cmov.html).

Today's programming languages haven't evolved to make things easier on programmers to describe non-scalar code. On the other hand, power constraints, and now security constraints haven't made things easier for hardware to efficiently execute scalar code. Perhaps AVX-512 is as naive a bet as Itanium, if not it might be just the missing piece compilers need that they didn't have twenty years ago.