It's hard to say until we see how the future CBA and free agent market goes. As things currently stand now, for sure, why would anyone want to be a 31 year-old FA? Another reason why Boras is probably pushing for 2 or 3 years (and some frontloading to boot). Not only will Harper still be young but they can react to the new CBA.
You never know where Machado's head will be at in five years, assuming he is indeed still on the Padres at that point. If he's still putting up great numbers he could use the threat of opting out to renegotiate, or, a la A-Rod, to get traded back East. If not to the Yankees than the Marlins.