If there is to be a promotion/relegation system, then there has to be more incentive to win and gain promotion than to lose and be relegated. You would rather have teams fighting to stay in the same league than to be relegated. The only incentives at present is cash and the increased quality of players.
Another option if a team is relegated, is to remove by sale or even better, reduced sale, 4 or 5 of the most expensive players as a relegation penalty.