Nothing and that's exactly what would happen if it became popular.

You'd get @kanyewest7, @kanyewest289, @kanyewest3058, etc.

Also, the younger you are when you join the network, the fewer posts you get per year. If you join as an 18 year old, you have a little over 1 post per year remaining. If you join at 90, you have 10 posts per year, etc.