You are taking a hilariously simple approach to what needs to be individual analysis. For instance 2016 Pillar and Tulowitzki were elite defenders, which means that they still posted a combined 6 WAR while largely being forgettable hitters. 2017 has seen both of them decline defensively, and since both are well within the age of defensive decline, expecting them to go back to being elite defenders in 2018 is nothing short of wishful thinking. How many defensively elite 33+ year old injury riddled shortstops can you name? Probably not many, and since there is no reason to expect that either Pillar or Tulowitzki will improve enough offensively to offset this defensive decline, you are more or less looking at the same thing next season as you are seeing this season. So while Tulo can certainly go back to being a ~100 wRC+ hitter, the fact is that he is more likely to be even worse defensively than he is today than he is of replicating his 2016 defense. Russell Martin has actually been better this season than he was last season, the scary thought is what happens if he takes a big step back next year...which again is entirely plausible as he is 35 next season and is therefore a ticking time-bomb. 2016 Donaldson was a 7.6 WAR player, what you need to understand is that even if he bounces back next year, it likely means a bounce back to a ~5 WAR season versus him going back to being a super-elite player (these days are likely gone). So there is another 2-3 WAR you have to find elsewhere.
When you look at individual analysis, you quickly find that you are stretching into pure wishful thinking when fantasizing scenarios wherein everything somehow goes right for us next season. 2016 also featured an unlikely season wherein we went the entire year without experiencing any significant pitching issues. As you can see from this season, expecting this type of luck on a yearly basis is foolish. What are we predicting for JA Happ next season BTW? Is his FIP-beater magic going to wear off like it did for Estrada this year?