The main issue with “counting” acharonim is as R’ Schachter says, that there was a “psak hamekubal” and the Mishnah Berurah changed it. Let’s say for example that the Chayei Adam had access to twenty acharonim and the Mishnah Berura had access to 40 acharonim and therefore overturned many of the Chayei Adam’s conclusions. Well let’s say that I have access to 60 acharonim, so now I overturn the Mishnah Berurah’s conclusions. And soon someone will have access to 80 acharonim and he’ll overturn my conclusions.
People who lived in Europe at the turn of the century, have testified that the Mishnah Berurah was not followed – even in Radin. Similarly, the Gra was generally not followed, even in Vilna, except by his talmidim. In fact it’s very likely that the Mishnah Berurah contributed to the Gra’s popularity.
I didn’t say that no great mechabrim were not practicing Rabbis. It’s just that one of the reasons why many poskim favored the Aruch Hashulchan is that in a choice between two works which serve the same purpose, it’s better to use the one which was written by an actual practicing Rabbi.