Would you say that if Harper's 2015 seasons was like the rest of his career?
I think that is predicting the future.
Many have tagged Harper as a future HoF. And that might happen (or not). He has the ROY, and MVP, 4 time All-Star, Silver Slugger and is 23. He needs about 8-10 more years approaching 2015 levels to be HoF or 15-18 more years approaching 2016 levels to be HoF. He has a career dWAR of 0.2 (Bref). He's 23.
And it might happen with Freeman as well, but for it to happen for him, he would need about 12 more years like this year. He has a #2 in ROY, a 5 and 23 in MVP, 2 time All Star, career dWAR of (-4.6). He is 26.
There is a lot of years each has to go to be considered best of All time. At one time Dale Murphy was the best player in the game by a significant margin and he isn't in the HoF and won't get there. A lot can happen.
But, I think a big component of value that is often overlooked is
player potential. People think Harper's potential is better than Freeman's. That doesn't mean he will have a better career. And players get paid for potential all the time. Take Heyward as a prime example.