I don't believe towing will shorten the trucks life. The power train doesn't isn't going to age appreciably faster just because of that kind of use.
With modern vehicles, it's more likely to be the things that aren't related to the power train that will start failing with old age and lots of seat time.
If you only use your truck for recreational towing, those types of failures are going to be a long ways down the line.
By the way, today's modern, so-called "1/2T" trucks are a lot more capable than even the diesels of yesterday. My 1993 RAM 250 Cummins powered truck was only rated to tow #7500!
