I explained this to you last season, why he would get 12-16 million per. He is a ~250 hitter for his career outside of Coors field and he was the 2nd worst defensive CF until he got to Wrigley. He started off hot as shit this season hitting ~350 for the first month. That injury probably saved him from regressing to a ~250 hitter this season, just like he ended up last season.
He also can't stay healthy. He has played more than 135 games twice in 9 years.
CFs who are on the wrong side of 30, are not particularly good defensively, ~250 hitters outside of Coors, and only play ~130 games a season don't usually get 20 mil per season. His value will probably pretty close to where it was last season. The only way I see his value going up is if there is no compensatory pick assigned to him. I don't know the rules well enough to know if the cubs can put a qualifying offer on him again if he opts out.
Fowler is an above average defender this season according to both DRS and UZR/150. As for his average, does that even matter? He's a .366 career OBP. Last year there were only 23 players in the league who had better than his "average" .366. Even if you want to suggest he's a product of Coors, the past 2 years he's hit .263/.364/.435 in chicago and that includes a horrid first half in 2015. He's been the 8th most valuable CF in terms of fWAR the past 2 years at 6.5. It's easy to sit here and say well he's 30 and you don't pay players over 30 but do you see how shit MLB CF's are? All those things you throw at Fowler also apply to Denard Span who got a 3 year $31 mil deal. Span's hitting .265/.336/.362.
As of right now, there's 17 CF with more than 200 PAs and a wRC+ over 100. Those include
1 Tyler Naquin .324/.391/.614 2.7 fWAR
2 Mike Trout .316/.428/.547 6.5 fWAR
3 Yoenis Cespedes .292/.364/.551 2.4 fWAR
4 Dexter Fowler .291/.400/.486 3.3 fWAR
5 Jackie Bradley Jr. .292/.371/.528 3.9 fWAR
6 Marcell Ozuna .295/.349/.509 3.3 fWAR
7 Ian Desmond .294/.346/.502 4.2 fWAR
8 Joc Pederson .239/.332/.478 1.8 fWAR
9 Cameron Maybin .325/.394/.398 1.1 fWAR
10 Brandon Guyer .248/.353/.413 0.6 fWAR
11 Odubel Herrera .284/.365/.417 2.4 fWAR
12 Ichiro Suzuki .317/.388/.372 1.2 fWAR
13 Jon Jay .296/.345/.407 1.2 fWAR
14 Charlie Blackmon .299/.363/.461 1.5 fWAR
15 Adam Jones .269/.314/.456 1.3 fWAR
16 Trayce Thompson .225/.302/.436 0.5 fWAR
17 Travis Jankowski .249/.365/.325 1.9 fWAR
If you're a team who's looking to compete for the playoffs, how many of those guys would you rather have than Fowler? Only Ichiro, Herrera, Maybin, Bradley Jr., Cepedes, Trout and Naquin have better OBP. Clearly Ichiro isn't a long term solution. Herrera is basically a younger version of Fowler but then again the Phillies likely aren't getting rid of him. Maybin has a good triple slash but he has no power. His .074 ISO is Juan Pierre levels. Cespedes isn't really a traditional CF. He's more a corner guy. Trout is obviously Trout but going no where. Naquin and Bradley are good young players neither Cleveland nor Boston would give up.
I get the argument Fowler doesn't have the best track record with health. And surely that's why no team is giving him a 7 year deal. But even missing a full month this season he's been worth 3.3 fWAR which if you use the $7-8 mil per fWAR figure that typically is associated with FA prices, that's worth some where between $23.1 and $26.4 mil in value. Alex Gordon got 4 years $72 mil last year($18 mil per) and he's a career .265/.346/.429 hitter in a corner OF slot albeit a very good defender there. Fowler is a career .269/.366/.423 hitter in CF albeit probably slightly average to below average depending on who you ask/what you look at.
Whether or not Fowler gets $20 mil/season isn't really my point. Teams make great bargain contracts all the time. They also go and pay $70 mil over 5 years for a 4.03 ERA Ian Kennedy. The point is that's what Fowler is actually worth. Think you can easily claim there's only 10 CF in the league who are at equal or better value to what he provides.