Yes, but I would claim that was when they started going to pot. LONG before the PC era, HP made its own line of computers, which were seldom seen outside of labs, and even labs were much more apt to have a PDP-8 or PDP-11. Their move into commodity computers and peripherals moved them from a situation where they were building the highest quality products available, into one where they had no "moat". After all, who would buy a spectacularly well built PC early in the PC era? After two years you would have wanted to throw it out regardless of how solidly it was built, because back then PCs doubled in performance nearly every year.

There would never be a moat around mainframe computers as commodity Intel chips became more powerful. SUN, DEC, Stratus, Irix, etc all fail to commodity PCs.