The team did Ryan so wrong throughout the years with constant offensive switches and bad head coaching and horrific O-Line play that I feel if we let him go and he goes to a competent franchise then he will do great.
And that could very well be true, but the alternative is to keep him and pay him an amount of money that keeps the team from improving elsewhere on the roster under the cap, with no guarantee that he'll play any better than he has already.
If he plays no better than he has already, and the team can't improve elsewhere on the roster due in part to salary cap restrictions imposed by Tannehill's salary cap hit, it'll be highly unlikely that the team can compete for a Super Bowl.
The article I linked earlier in this thread does a good job of illustrating Tannehill's effect on the team's salary cap.
Also consider that you're implicitly stating here that Tannehill is dependent on high(er)-quality surroundings for a high level of play on his part, whereas there are quarterbacks in the league who have the capacity to surmount poorer play around them, and who one could argue better deserve the kind of salary that prohibits the acquisition of talent elsewhere on the roster.