Adam Wainwright had TJ like Darvish has and the timeline is very similar. He had strong seasons at 31 and 32 and has been middle of the road since. Sabathia cratered at 32. John Lackey had TJ and is still going at 38, but he has not been a number one starter in almost ten years.
Wainwright basically had peak seasons at age 31 and 32. (And he's an August birthday like Darvish, so that means he actually turned 32 and 33 in those seasons )
John Lackey almost had a career best season at age 36 (also after TJ).
These guys still fit the general overall trend of being worse in their 30's than in their 20's, but that doesn't mean they were valueless. In fact, both proved worthy of the long-term deals they received around age 31.
Of course, Darvish is a different person, and his future hasn't happened yet, so Wainwright and Lackey are no guarantee of Darvish's future results. But in no way do they support your position either, that a long-term deal for Darvish should be avoided like the plague.