Well, I have no idea, how often your theory is correct, but it's not true in my gaming circle:
Our current (4e) D&D group includes several players that have been playing together since 1984 using a variety of systems, including D&D, AD&D, DSA, Midgard, Shadowrun, Star Wars, Runequest, Ars Magica, Earthdawn, and several others that didn't 'stick'.
Most campaigns ran for several years before they fizzled out. Our most recent 3e campaign took over six years to finish.
In my experience all it requires is one gamer that enjoys trying out new stuff for the rest of the gang to be willing to give it a shot. And if you're regularly exposed to new systems, you learn to appreciate the differences even if you eventually find your favorite system.
In my circle of friends, what happened was that almost everyone picked a different favorite system and started a gaming group to play it. There's a huge overlap in players of different groups and I play in at least two groups at any time.
It may also have helped that we founded and registered a gaming club that was very active for a couple of years, working with RPG stores, organizing cons, etc. to expose new players to new stuff all the time.
