I'm fairly surprised that Andy Pettitte is in fact retiring, assuming news reports are correct and he doesn't wind up returning. It's very rare these days (post-free agency) for a player to retire under age 40 (this would be Pettitte's 39 year old season, if he pitched) and still league-average or better. The most obvious comp is Mike Mussina, who hung 'em up after a very nice 39 year old season. Mussina had a solid shot at 300 wins...we'll see if either one of them makes the HOF, but both almost certainly hurt their chances by leaving early.
Who else has quit early since the big money came in? Will Clark is an obvious one. The Thrill was done after his 36 year old campaign, in which he moved to the Cardinals at the trade deadline and had a 426 OBP/655 SLG two months and an even better postseason. Now, Clark probably would not have made the HOF no matter how long he played, unless he managed to hang on long enough to get 3000 hits (he needed 824, probably another six years or so -- not impossible, but not all that likely). He was a terrific, and I think underrated, player, but he didn't do the sorts of things that HOFers do, so even longevity probably wouldn't have been enough. I always assumed he just didn't like baseball all that much, although as it turns out, he's stayed around the game, so what do I know? I'd love to see a list of similar cases.
My assumption is that the all-last-year team from pre-1975 would be a lot stronger than the team from post-1975, and even more so if we could focus on only the players who retired voluntarily. But perhaps we've reached a point in which big stars aren't given big contracts after they stop playing like All Stars, and so the marginal value of that last contract isn't very important compared to what's already in the bank. It would make sense for the teams to do that -- but it would be too bad for the fans.