steel frames for racing used to be finished after a season
I suppose bike companies have invented a steel which does not obey the laws of physics then Very Happy
Erm, no. Just very, very thin walled tubing that can be damaged by the scrapes and knocks it'll pick up in a season's pro racing. Very high-end frames couldn't have tubes replaced as they could only take the heat from brazing once before becoming brittle.
Remember that pro-bikes weren't the same as the ones you could buy as Joe Public - even if the bike in the shop has the same paint job and team replica stickers whacked all over it. They were built to last a season of hard use and that's what they got.
An old frame is probably fine for retro-riding around on but if you subjected it to the stresses of racing then it would break. Add 20 years of internal corrosion to paper thin tubing and the failure you experience might not be entirely 'soft' Think about the consequences of a fork crown failure or the headtube being ripped off when you drop the front wheel into a pothole.
For normal frames - yeah - steel should last a long time. Pro frames are not normal frames.
Raleigh Ozark (1987)
M-Trax Ti-1000 (1995)
Dynatech Roadie (1996)