For me it was clear when they removed the Turbo switch.
I think some overclockers still make their own, but yes, this clearly marked the start of the decline of personal computers.
No doubt the 286 architecture peaked back then. I guess his assumption of no backward compatibility is where it went wrong. Intel kept piling on new architectures while keeping the old for application compatibility.
This is exactly it. Also the reason why IA64 failed, and we're using amd's x86-64 instead.
In more recent times, the argument is less relevant because the compatibility layer is software-side, see the mostly painless x86 -> ARM transition by Apple.
In 1992 (just over 10 years after the IBM-PC was introduced), with version 3.1, Windows started becoming much more popular than MS-DOS.