I'm thinking of a certain right-handed pitcher who was 28 years old, and a veteran of several major league seasons.
This pitcher had two very fine seasons at the ages of 24 and 25, and then seriously regressed in his age 26 year.
In fact, it got worse. When he was 27, he had a year interrupted by injury and a move to the bullpen.
And then in the following offseason, he signed a free-agent contract with the Chicago Cubs, to almost universal derision.
Sounds a lot like Jason Marquis, doesn't it?
The pitcher I'm talking about is this guy -- and yes, you'll have to look at the link to find out who he was. He wound up putting up two very fine seasons in a Cub uniform, upon which he was allowed to leave via free agency... after which he sucked again, but that's another story, and as Cub fans we didn't care because he was sucking for someone else.
Or how about this guy, who the Cubs acquired by trade after he had put up two pretty bad seasons elsewhere; he became a key cog in a Cub playoff team. At the age of... 28.
Or this guy -- who was almost on the scrap heap after four mediocre years with his original team (with ever-increasing ERAs and WHIPs), then traded for a middling outfielder, and wound up having five great seasons for his new team, including being the key cog for a division champion.
The point here is that before we judge Jason Marquis, shouldn't we at least see him throw a pitch in an actual game? These are only a few examples of situations where the change of scenery really, really helped the player in question, and right off the top of my head; I am sure there are many, many others.
Players sometimes "figure it out", even when all the "statistical evidence" points against them.