Everything can be saved. If there is a sense of saving an overdriven bike by caterpillar, that can be questioned. But of course it's much easier to start building better frame than the one that has a lot of work before you even get to the point of building it back again.

Sometimes you just have to make a decision whether to save or not to save something. And personally I see a lot more trouble to fix something for my own use than to those that are sold onward. I give guarantee for the bikes I sell on, so can't take a risk picking up a bike that is dangerous. But you have to rely on the eyeing the bike on location. There are a lot of things that can be bad in the bike what you can't see visually by just looking. I'll try to drive those bikes first myself to see they work properly.

One thing that tells something with geared bikes is that on smallest cog(s) the chain slip when pressured with hard pedaling, and it means that the frame is giving up. It is not as sturdy anymore than it was. That can't be seen just on bare eyes, except looking how worn the cogs, chain or derailleur pulleys are. But somebody might have changed them at some point.

So happened with a beautiful Peugeot. Paint job was still pristine. Did everything, changed all I could but the chain was skipping/slipping. So had to turn it to be single speed, unfortunately. And with a quite light gear ratio. When giving a lot of pressure to pedals, the chain was skipping annoyingly.

Steel frames need to be straighten slowly using time. Can't rush. Like the one here.