One-and-done curiosity

There has been a steady stream of one-and-done players into the NBA for several years now. Something puzzles me about that. At some point, don't the one-and-done players who are now have several years of NBA experience and the non-one-and-done players who showed they were really good begin to choke out the room for raw freshmen coming in? Are we to assume that each freshman class is better than the one the year before?