I would counter that most players aren't 'great' in their late 30s, no matter which position they play.
But I agree with the latter half of your post, by and large, although to me Trout represents a special case.
All-time greats usually are. Unless they play certain positions. 1B, center, and catcher are the main ones. Center and catcher because of how demanding those positions are and 1B because the general body type of most 1B isn't going to age well. I agree with Zito. Trout should be moved to right if they want to increase his chances of being a really long career and still be good.