Fister fastball is 86 and falling they last two years. Unless they have something to bring it back up to 88-90 he isn't the type of pitcher a contending team should go after. Fister was a guy that works low in the zone and gets ground balls. At 86 that isn't going to work. He will have to pitch up in the zone in an incredibly small area to get weak contact. Let another team feel the pain. Even Maddux reached a point where he couldn't get by at a low velocity.
Fister situation is more complicated than just throwing 88-90.
First, Can't lump together 88-90 velocity, as you do. Fister at 88 is fine. Doesn't need 90. Has rarely been 90 in his career. 86 is another matter. Even though Jered Weaver has done fine at 86.5, tough to do. Fister has been hurt in 2015, so maybe can tick up a bit. Hard to say. But, other stuff too. Fewer ground balls this season hurt him. Walks up a bit. Combo of things, mainly his health. Not just velocity.
Also, don't agree that going from 88 to 86 means that a pitcher has to work up in the zone. Fisher's command was not as good in 2015, along with lower velocity. Just made more poor pitches, not because had to or needed to work higher in zone. Fister is generally trying to induce grounders and commanding bottom of zone. Always has.
Second, signing value depends on several things: contract cost and term; type of defense behind him, where he is expected to fit in rotation. Lots of teams will value him for the right situation. Not saying he's necessarily a good fit for Cubs but would not be surprised if turns out to be good value for somebody at right price and situation.
Not just a black or white thing whether he can get to 88-90.