Sure relegation/promotion is not preferable for certain (or all) schools that are already in the top tier by default. That's why it'll never be introduced into a sporting culture in which it doesn't exist (i.e. the US). But relegation/promotion is actually a huge net positive for the sport as a whole everywhere it exists. The value relegation and promotion provide is that they add excitement and importance to games that wouldn't otherwise matter. Illinois v. Indiana in football could go from a game nobody cares about to a game of huge importance. Imagine a scenario where the Northwestern and Illinois rivalry game also determines which of those schools would be relegated (or forced into a relegation playoff depending on how the whole thing is set up). That would be a high stakes competition!
On the flip side, imagine the excitement of being at the lower level and possibly having a chance to move up in the ranks. Could there be anything more exciting as a fan of a school like Kent St. or something. Again, I get it doesn't serve the Illinois self-interest, but on a basic, idealistic level, where sports is about meritocracy and the cream rising to the top, who wouldn't want a system where any team can, if they do the right things and just keep winning, force their way to the top?
It's all academic because there's no way it'll happen, but the reason people advocate for it is that it drastically cuts down on the number of games that do not matter and adds a ton of excitement to the sport as a whole, from top to bottom.