If done right, it should last years and years with the proper care. Sometimes, it is just easier, less time consuming, and maybe even less expensive to jsut replace the part rather than repair it. Other parts, like for the cab, need repair since a replacement cab isnt cheap or practical.
Another thing...about bondo....I get picky when I have to use bondo. I dont use "Bondo". I use All-Metal (i think). Instead of talc powder, it uses aluminum. This makes the filler adapt to conditions like your sheet metal. It also doesn't soak up water like talc based bondo. Thats one of the reasons bondo products fall off after awhile. Thats me just being picky though. Just thought I should share.