Name: G.I. Jonesy
Put simply: tier slumming could be effectively eliminated by disabling strafe-jumping at low-tier. The fact is, no experienced player could live without strafe-jumping. Most new players probably don't know how to strafe-jump. By disabling such a vital function, the lowest of tiers would be unplayable for experienced players.
To continue the process of bringing in new players gradually, the next lowest tier could enable strafe-jumping, but disable some of the weapons and/or maps. Again, experienced players find little value in playing without a proper weapon and map selection. For new players though, it would make the game considerably easier and more fun. Plus, they could learn how to strafe-jump in a relatively safe environment.
With these types of changes, continuing up the chain until everything is enabled, we can adjust the depth of the pool while still maintaining a deep-end. Other than that, I suppose we could just tell everyone how stupid they are, and point to their IQ scores as proof.