Some aluminum frames are more robust than others. If you buy a well-built aluminum frame and take care of it, it will probably outlast you. If longevity is still a concern, take a look at buying an aluminum frame from a manufacturer that offers a limited lifetime warranty (Trek comes to mind). I have owned several aluminum frames and ride them a bunch. I've never had any of them wear out.

Regarding comfort, my wife has a Vitus 979 - it's an older thin-tubed aluminum frame. Super comfortable, not at all an "asshammer". In fact, it's probably one of the most comfortable bikes I have ever ridden. It's flexy, but if you don't mind a little chain rub on the derailleurs when standing, it's not too much of a problem. My point - aluminum isn't necessarily uncomfortable. Much of aluminum's reputation of being uncomfortable probably has to do with Cannondales from the late 80's and early 90's. Back then, those bikes were super stiff but quite uncomfortable. Aluminum frame designers have gotten better at finding the right balance of comfort, performance and durability. But unfortunately, aluminum's reputation for being uncomfortable has stuck around.