Much of what is required for a new vehicle simply doesn't fit within the envelope of the old body. Crash/impact standards wouldn't be attainable within that package either. There must also be economies of scale, e.g., the ability to use some hard parts (like an HVAC box, or turn signal switch) from another vehicle built in greater volumes to achieve a profit. Such parts are obviously not backwards-compatible with something made 50-years earlier.
I think the Bronco that's coming any day/week/month now is just as good as the U260, maybe even better. The Bronco Sport isn't for purists, and its unfortunate it made production before the "real" one.
The appeal of the U260 is it’s simple, honest design, which unfortunately does not fit into our current culture. The masses want big, bad, look-at-me trucks. Maybe the new Ford Maverick will be closer to the simplicity of the U260 design, but I have my doubts.