Well, as I was born during the Truman years, I feel pretty confident that any of my bikes will last for the rest of MY lifetime. But, I don't think anyone in their twenties would be realistic in thinking they will be riding the same bike every day for the next fifty years (even if they wanted to).
Most of my frames are over twenty years old, and seem to be in good shape. However, because my riding is spread among ten or so bikes, none of my frames get a lot of miles. And, my miles are very low stress miles, cruising slowly along paved streets and bike trails.
A younger, larger,faster, more powerful rider putting hard 10,000 miles a year on a frame is going to wear that frame out eventually. If it is a lugged steel frame, cracks can be repaired, and tubes replaced. Aluminum...well, it recycles rather well.
There are no guarantees on how long a given rider or a given bike is going to be around. It is easier to just ride a bike that you like, and when you wear it out, get another bike that you like just as much.