I don't buy that at all. The NFL complains all the time at draft time about guys who are 22+ years old already being "too old" and that's a knock on their draft profiles. They'd do just fine plucking out the best 18 year olds on the planet. They don't need all of them, just the toppest of top tier ones, and the rest they want can sit on practice squads or whatever -- again, they'll adjust based on the opportunity presented. Every other major sport in existence already figured this out.
If I'm the NFL, I certainly don't wanna wait around while the colleges slowly build towards axing roster limits, scholly limits, paying kids more and more to stay longer, all of that is competitive to them (somewhat loosely, but still 'competitive'). The more self-destructive the NCAA gets in terms of actually being a regulatory body over the sport, the more trouble that will cause down the road for the NFL. The NFL has never had a better opportunity than right now to overtake the viewership space that college currently occupies. Plenty of fresh, new money to be made.
I don't buy the idea that the NFL and NCAA are effectively unwritten partners. They merely tolerate each other but aren't beholden to one another.