Rust is a major problem for cars of that age. Sadly, poor quality steel produced in Italy in the mid and late 70s had some cars rusting after their first winter. Salt on the snowy roads didn't help either. Undercoating wasn't a true solution to the problem of rust. Only the advent of less rust prone materials finally offered a proper solution, so today, rust is a rarity, but still occurs when manufacturers have quality problems or initiate cost reduction programs without thinking the process through. The press quickly picks up on this, but the poor customer is stuck with their lemon.
My first car was a nice, yellow 1972 FIAT 124 sport coupé. Had to learn how to a choke. My dream car back then and until recently was a Porsche 914. But finding one without rust or without stupid body work of spoilers, skirts, loud exhaust, etc is neigh impossible. I had a neighbor who loved VW beetles, most cars he owned had the problem of rusty running boards.
Anything mechanical can be exchanged or repaired, alas, rust is difficult enemy to battle.
