Well if he hits his bonuses that means he was doing something right and it's STILL cheap. As I've said before the only negative I can see is if he does play well you will either REALLY have to pay next year or lose him and replace again. But that's for next season.
Now days QB's can play until they are 40 and even older and be effective. See Brady and Brees. Peyton was almost 40 before he retired, turned 40 that March after he won the SB. Only reason why he retired was because of his neck. He couldn't throw anymore or he would be like Brady and Brees. Rivers is 100% healthy and only 38. When it comes to all other positions, I prefer players that are 28 or younger that I agree with you with exception of Kickers, Kickers can play into their 40's as well.