There wasn't always a players union. In fact, it's quite a recent development given NFL history. What you say that now they don't need representation was not always true. Hence, the union. There also never was a retirement fund or health benefits before the union. It is those side issues that have kept the union in play. Not too long ago it use to be that football was a player's 2nd job and the season was shorter because of that.
Now teams' players are, team by team, decertifying the union so you may get your wish. I'm not sure it will work out as you describe. There will always be the Jerry Jones and Dan Snyder type owner.