Actually, many people have set PRs at Boston, including many world-class runners. In fact, my hunch is that many people who post on letsrun.com have set PRs at Boston. Perhaps we could have a show of hands . . . .
It is true that, among national-class male marathoners from the United States, Boston has not produced many fast times since 1983. That's not because Boston suddenly got tougher in 1984. Rather, there were a number of other changes that occurred around that time -- in the sport generally and in the Boston race organization in particular -- including changes in financial incentives, such as prize money and appearance fees.
As I said before, I believe that Chicago is, for a number of reasons, a better bet than Boston for most people who are looking for a race that produces predictably fast times. That doesn't make Chicago an "aided" course, however. Chicago is simply a reliably fast "unaided" course.
Whether or not performances on "aided" courses should be accepted for various qualification purposes is another matter. It need not be an all-or-nothing proposition. As an earlier poster indicated, one could certainly draw the line at a point that accepts performances at, for example, Boston, while excluding performances at, for example, St. George. Although there are some real problems with doing so, you could even consider certain factors, like tailwinds, that vary from day to day on a particular course, just as is done with 100-meter races and long-jump competitions. What bothers me is the notion that, as long as the length of the course is reasonably accurate, anything goes. Both literally and figuratively, this creates a "race to the bottom," in which race organizers compete for customers with courses that are increasingly aided.
By the way, although we've been focusing primarily on the legitimacy and significance of performances on downhill "marathons," one of the most extreme examples of an aided course was the old Mardi Gras Marathon course, which was about as flat as any marathon course in the world, but consisted almost entirely of a straight-shot run over a bridge. If you were fortunate enough to get a nice tailwind on the appointed day, it was PR City.