Exactly, it makes sense to have the NCAA rule on whether or not a player is academically eligible to represent their institution in athletics. But any rule on the number of years is arbitrary. An age limit to protect the interests of athletes might make sense, and creating different levels such as undergrads and post-grads could make sense, but is anyone going to field post-grad only teams? Of course not.
So to me they need to hang their hat on enforcing the academic side of things only, and you can make an argument that 5 to play 5 makes sense academically because if you're in good standing taking at least 12 credits a semester it shouldn't take you more than five years. They can also make a flimsy argument that the spirit of 5 to play 5 is to keep it to primarily undergrads, with some wiggle room for kids who excel academically to get a post-grad degree in within 5 years.
Where it gets really interesting is that you could also make an argument that anyone academically enrolled who is not a professional athlete should be eligible, which would lead to guys like Eric Dungey or Gerry McNamara leading 10-15 year college careers. Someone like that who clearly has higher earning potential in college and little/no NFL/NBA future is highly incentivized to sue at some point, the trick is finding a 22 year old who is level headed enough to realize they don't have an NFL future and can make more in college.
You could make a pretty strong argument that someone in the mold of Dungey right now would be best served by getting every degree possible while making $500K-1M playing college football until he's into his early 30s. Ultimately, what's the harm in that? It's absolutely not the model we're used to, and a lot of us wouldn't like it, but is it wrong? Are we going to make the argument that at a level where kids are paid seven figures to play, it's unfair to the 18 year olds who haven't developed enough physically to compete with "grown men"? All the while a bunch of them are a year or two removed from the NFL? I don't know, I really don't.
This is all a slippery slope, but college sports had been streaking across the quad towards that slope on a rainy day for a while, and now they're sliding down it buck naked.