Certainly there is no economic rule, however, I think that most people generally believe that for most of human history it has been the case that at least in the medium-to-long-term, top line growth does accrue pretty cleanly to the middle 50%.

There are certainly some people that are starting to wonder if this is no longer the case but personally I'm always pretty skeptical of "this time it's different!" arguments so am unconvinced by the evidence to date. Who knows what the future will hold though!