A.B.C.
I think we can see the same thing parallelled in hockey. If you look back at things like the '72 Summit Series, or the'87 Canada Cup, it's all about the Canada-Russia games. However, if you ask most Canadian hockey fans what match up they get "up" for the most, it would be Canada-US, fuelled in part by the '96 World Cup of Hockey, as well as Salt Lake City. While Paul Henderson's goal was significant in that it occured against the Russians (in much the same way that I'm sure Stransky's drop-goal in extra time of the '95 Rugby World Cup had an added impact because it came against New Zealand), there's not the same sense of occasion when Canada plays Russia internationally. Even looking at the local level, in recent years the hockey version Battle of Alberta has been overshadowed by Oilers-Stars match-up (well, I guess Dallas could sort of be Calgary really south). Football is similar, in the sense that Montreal is starting to be seen as more of a rival than the Stamps. Mostly because the Stamps suck now.
Which I guess leads me to my point. I think alot of good sports rivalries are a combination of a shared history of enmity and competition, with the proviso that the other side is actually competitive. The reason why South Africa, Russia and Calgary have sort of fallen by the wayside, is recently (though, in a couple of cases, this trend is reversing) they have all been less competitive with the primary rival than have the likes of Australia, the States and Dallas and Montreal. I think that a deep-seated rivalry always has some impact though, and can outlast others based merely on competition. For example, we've seen a cooling of the Detroit-Colorado rivalry in the NHL, mostly because they haven't played eachother in the playoffs lately. That would be a rivalry based solely on competition (and the fact that Claude Lemieux is a weasel). By the same token, a game against Montreal would be meaningless if they weren't as good as they are now. However, because of the history attached, and the close proximity, a game against Calgary always has some special-ness attached to it, even when Calgary isn't competitive. In that sense, I think that even when they may be overshadowed by rivalries that are more in vogue, the traditional ones always have a remnant.
Even though the Stampeders were the second worst team in the CFL last year, it was still exciting to shut them out in the Labour Day rematch, because they were Calgary. It wouldn't have been as exciting if it were Hamilton. Indeed, it wasn't as exciting in 1997 when the shut out Montreal (before Montreal went to the next level, and had the added intrigue of being helmed by a former Eskimo coach). So I'm proud to cling to the long-standing rivalries, even when they may not be in vogue, as they've proven to have greater durability than the other sort.