That would mean a least a hundred wizards before McCoy on the seniority list.
That's simply not possible, as I cannot see how no one among those one hundred would accept a position as prestigious and powerful as SC member.
I dunno -- I'd imagine most wizards in their second or third century have a lot of things going on in their lives. Why would they want the hassle of Supreme Council work? Keep in mind how often the SC members seem to be working at Edinburgh, or on whatever new crisis is facing the White Council. Think of all the paperwork.
Remember that the "hundred or so" wizards ahead of McCoy on that list didn't show up to the meeting at all. And why would they? Some silly war with the Red Court? A passing nothing to a three hundred year old wizard.
The White Council wizards we meet are all either Senior Council members themselves or wardens (or occasionally random audience members at meetings). They are *exactly* the wizards who *do* care about the White Council. I have to believe the vast majority are more like Elaine, except they pay nominal lip service to the Council when they think about it at all, every few decades.
Heck, Ebenezar was involved enough to be the Blackstaff, and he still didn't want to be on the SC -- he'd turned it down several times. He only joined at this point because Simon was dead, and if Eb hadn't stepped up, Harry would have been killed. Not hard to believe that most wizards of Eb's generation feel the same way. Set in their ways and happy to go about their own business without all the hassle.