The contribution of the Premiere RB has diminished significantly.
Not to mention the nature of the position's effectiveness is usually tied to spending money on a offensive line to supplement said back. That's even more money for a position that contributes less and less to team each year. The receiving core's effectiveness however involves less, is more crowd pleasing and is the premiere source of offense in today's game. When it was a running league, backs ate good and receivers were underpaid. Now the tables have turned so all the Chris Johnsons in the league need to enjoy those last big money contracts.