If anything that rule applies now, certainly not in the 90's.
Interesting conversation in this thread, I'll throw out my quick opinion.
I think what the issue is is that back in the 90's game publishers still hadn't figured out what sold the best, and of course that is the fps and fps multiplayer, nothing comes close.
That's why you see such a variety of genres from top publishers from that era that are dead now, cfs, space sims, point and clicks, etc. They were fantastic games, but companies only care about money. The yellow brick road is the road always followed. And the funny thing is 20 years later we are still in the fps era, and it doesn't look like anything will change soon.
Something else to note is except maybe with the exception of il2, there weren't many cfs that were top sellers, they did well, but not overly.