The reason I say it simply that they will bet on themselves having big years and hope more teams have payroll open up and want them. At his point, neither has gotten an off they want. Maybe they never will because those days are gone. If Donaldson got 1 year and 23M either of them could get 25 or 26M.
Oh I get what you're saying but I just don't think there's a "big year" out there. I mean Machado hit 37 HRs and had a .297 average to go with the highest walk rate of his career. If you want to argue there's a little bit more skill there to unlock ok but to get substantially more like you are talking you'd have to have him go from being something like a 140 wRC+ guy to 160+. Additionally, you could argue that maybe he plays lights out at SS for some team next year and that adds some value but idk like... people know he was a gold glove 3B. Unless he's literally a gold glove SS next year I don't see his value improving. So, could he be better in some ways next year? Sure, but there's no way it's worth risking getting hurt. He could lose half a season and it will tank his value worse. And that's not even really talking about the potential for him to have a career ending injury however unlikely that may be.
I think your argument with regard to Harper is slightly more compelling.... but still not really. I just don't believe there's another 2015 season in him. I mean he hit .330 with a 19.0% walk rate and a .369 ISO. Those are basically Barry Bonds when he was on steroids numbers. Bonds is potentially the most gifted hitter ever and he never prior to 1999 had an ISO over .341 and only once had an average over .312(.336 in 93). Bonds did post rather regularly some pretty bonkers walk rates so that doesn't surprise me to see Harper continuing but combining all of those things.... i mean it just doesn't happen much. Even Mike Trout doesn't do it. His career best iso is .323. His career best average is .326.
So, if we can agree it's incredibly unlikely that 2015 season is out, then what's a realistic best case scenario? I'd argue something along the lines of his 2017 season(155 wRC+). One could argue his 13.8% walk rate that season was really low given the other seasons he's had. And while I agree with that, I think just saying that's a reason that there is more there than 2017 is wrong. If you remember, Maddon started pitching him like Bonds and it broke him. I personally believe that having a high walk rate is a very fine line because while it means you're patient... it also means you are going to miss pitches to hit. To an extent, I think that's Schwarber's problem. He'll work a count great but when you get to 2 strikes it's hard for any player to hit. So, you'll often see more walks lead to more K's. And in Harper's case, he sorta had both k rate and bb rate good in 2016.... but his BABIP was .264 which lead to a really disappointing year. There's a lot that could be but I believe it's a case of him putting weaker contact into play because of deeper counts.
As for what that's worth, I mean Mike Trout is making roughly $34 mil the next two years. Harper isn't Trout. It's not even close. Trout is a career 172 wRC+ to Harper's 140. Trout plays CF and Harper some what poorly plays RF(may need to move to LF sooner or later). Cespedes is a some what similar player. He's making $29/$29.5 mil the next two years. Stanton is also a similar player though I think he's a better defender. He's owed $260 mil over the next 9 season but if he doesn't opt out in 2020 Miami is paying $30 mil of that. So, that's like $25.5 mil a season as a similar real world value for a guy on the open market. realistically speaking, I just don't see much more than $30 mil a year out there.
So, if we're being realistic here, my view is that Harper can probably get more than $25 mil a season pretty easily. I'd even say $27 mil/season is likely. I'm not sure he realistically gets more than $29 mil/season though. Like Cespedes' deal works because it's short term. He signed at 31 for 4 years. If Harper is signing for 8-10 years a team is going to have to factor in years he's going to get hurt and miss time and the potential that he does what Heyward did.
Just as an example, the stats types viewed Heyward as a potentially $300 mil type contract based solely on value before he signed. Heyward actually got 62% of that and some people think that contract is still a giant albatross. If you think Harper is a 5 win player like he was in 2017, that's some where between $40 and $45 mil value but if you discount that 62% you're looking at $24.8-$27.9 mil. And all of this assumes that the market hasn't changed(it looks like it has). $200 mil players last year had trouble getting $200 mil. That hasn't changed this year. I doubt it changes next either.
That's why all along I have told people they are crazy talking about $400 mil contract let alone $350+. if I'm Boras, what I'm trying to get for Harper is a deal such that the first 2 years pay him maybe $35 mil a season with an opt out after 2. That way he can effectively say Harper is the highest paid hitter in baseball though it's kind of fudging with the numbers. For a team to do that, they are going to want to get his AAV down a lot. For the sake of argument let's go the top in $28 mil. You could realistically do something like the first two years at $35 mil + an opt out, then the next 4 at $24.5 mil a pop for a total of 6/$168. My guess is Boras would probably want that number to be a 2 so you'd likely need to add 2 more years or 8/$224. That's quite a bit bigger than Heyward. It's a higher AAV than stanton's 13/$325. It's slightly less AAV than Cespedes but if he's good he's going to opt out after 2 years anyways and he has the long term certainty if not.
That's how I see this playing out and honestly I don't think that is really going to change much next year.