I disagree. The Mainframe era is evolving, not ending. How many people still have and use a mainframe when they interact with their bank? PCs are filling a growing demand for an OS on a smaller form factor, but the traditional mainframe market is still alive and kicking, its just not "cool" anymore; it's seen as more of a utilitarian tool than a novelty at this point, whereas PCs currently retain the novelty factor, thus their pervasiveness at BigIronExpo.
Yup. I actually recall people making this argument back in those days. And they were right in a sense: big iron still exists in its niche, after all. Regardless, the PC revolution was still a massive shift — large enough to justify the rise of the PCs as heralding the demise of big iron.