Most pitchers start getting bad at around 32-33. Especially if you are a power pitcher.
John Lackey known by many to be only mediocre, has had a very good career if you think about it. Jamie Moyer had an amazing career if you think about it . Guys with good control, can throw at least 3 pitches for strikes and are relatively injury free.
Look at Jake Peavy. He is only 33, but really struggled with us before being traded. He managed to have a good ending with San Fran but had to try really hard. Don't think he will ever be the same again.
I consider Shields possibly able to pull another Lackey. But you don't give him 20 million a year for Lackey like production. After 2 seasons likely his production will go down and won't be able to pitch as deep into games.
It kind of sucks we don't have another Lester or even Bucholz young ace like pitcher to depend on currently. Though there is high hopes for Henry Owens to be that guy.
Redsox should not pay Shields 20 million a year deal for 4-5 years. I would do it for 2 , but he wants a long deal.
So bottom line is, just stay put i guess (most teams will not trade their ace or even #2). Don't have a good feeling about Hamel and looks like he is on his way down. Hope Porcello becomes ace material, Owens becomes ace material soon, Bucholz can regain form/stay healthy and/or our offense can carry this team. Might have to pull a "whitesox without a true ace" to try to win the world series this upcoming season