The problem that will arise if this keeps going is the Owners will have to increase their payroll when the Agents of the non-superstar players start whining that why is my client not making what a College kid is making. When it hits their pocket books by having to pay their 2nd and 3rd string players more, it'll catch their attention.
1. The NFL has a salary cap. The amount of that cap is determined by an equation (tied to media revenue) voted on by the Players Union.
2. The NFL has ZERO problems telling a player to go play elsewhere if they don't like the rules or the pay. Where is a second and 3rd stringer going to go? For that matter, where is a 1st stringer going? Nowhere, because the NFL has a monopoly, and NCAA has an eligibility window.
3. Once that eligibility window expires, you play by the NFLs rules, if you're lucky enough to get a roster spot, or you go get a regular job. For every roster spot available, there's a 100 players willing to step over you to take it. Simple supply and demand. So you return for your senior year because you make more money playing in college, what happens when your senior year runs out?
4. Once a player is in the NFL, they can't return to college. It isn't a two way street. There's no leverage for an NFL player to use college money as a bargaining chip.