I am interested in debating this, because I find this interesting and I have heard these arguments by established guilds against new guilds before. My hope is to see if I can broaden peoples opinion then propose a solution to what is seen as the diluting process.
Consider two parallel shards, one with 3 RP guilds and one with 10. What I think you are arguing is that, the shard with 3 guilds will have more members in each and the possibility that the shard with 10 guilds would have more RPrs overall is irrelevant, because they are not "real rpers" or they are too spread out.
Maybe, quality and concentration is better than quantity and diversity. For argument, I'll concede that point. But, how would this be achieved - establishing some RP guild registration committee who automatically denies formation of new RP guilds who are not one if the established guilds? This is not the way to stop the real or perceived diluting process. There is no way to stop budding RP community leaders from trying out new RP ideas.
My serious recommendation is, use a mechanism that already exists for guild communication and cooperation, the Alliance. Make 2, Good/Evil or 3 Good/Neutral/Evil type alliances, for example. Have the established guilds join these alliances. Then as new smaller guilds come along they can join in on the Big Guild feel that the Alliance system gives or risk being marginalized further and disappear.