The quality of frame construction is so good now that steel or aluminum will make little difference long term as your Cannondale has proven. I suggest focusing on components for the "last bike". Get absolutely the best drive train and wheel components you can afford. Absolutely avoid a suspension fork. Cheap forks are just that, cheap and garbage long term. Top notch ones are great, but will need service over time, and 10 years from now parts may not be available for rebuild.

As an aside the 1995 F500 Cannondale I have still works perfectly, even the Headshock! Shimano XT components are the workhorse of the lineup and are my favorite for mountain bikes or touring bikes, and are all original on the F500 and work like new. Of course I take very good care of it, so I expect it to last 30 years.