Back in my racing days we just about all replaced our bikes every season. The official line was that the frames had gone soft. I personally never noticed any such thing, but went along because it was nice to discard the banged up old race bike every year.

I've talked to a couple of mechanical engineers about the issue over the years. The first one told me the same story as has been mentioned above: no way is a bicycle going to get soft from use. If anything, it will work harden and get stiffer.

The second one said the same thing but with a twist: he knew what he'd been taught, and he believed it right up until one of his old steel frames started to, well, soften up. Soggy bottom bracket, head tube given to wobble, lack of "liveliness". And he's at a loss to explain it. He's come up with several theories, and he doesn't like any of them. I've had the same experience with a 30 year old frame that had seen a lot of racing, was hung up for a decade and then pressed back into somewhat lighter service. The frame is definitely softer now than it was when I first started back riding it a couple of years ago.

Having said that, I've never known a steel frame to become unrideably soft, unless it was broken. My "soft" frame is still perfectly functional for recreational riding, but when I hammer it at the "race rides" it isn't so good anymore.

HTH!