Modern cars still rust, but where the rust free 10-15 year old car was the exception for cars of the 50s 60s and 70s, the rusty 10-15 year old car is the exception today, and it is usually mouch more isolated, one or two spots, rather than pervasive, but if you live in the rust belt take a look at the 15 year old cars still driving around, some are starting to rust.
Our collector cars were "throw aways" for a lot of owners back in the day, tops were left down in the rain, dirt wasn't washed out of the wheel wells, they led hard lives for the most part. I think Donald Healey himself said they were designed to last about 7 years or so.
The life of a collector car is much easier, my brother and I both restored Triumphs 20 or so years ago, he a TR3 and me a TR4A, we repaired all the rust, but we didn't do that great a job, but 20 years of sheltered life as a collector car and no rust issues whatsoever on either car.