Like all things vintage, parts do give out. I've had it happen on other bikes and old cars that I've owned. It's very possible that I unwittingly accelerated the demise of the spring by pushing it too far forward for its age. However, after nearly 40 years of use, there had to be some wear on the spring that didn't tolerate the distance I pushed it. For my bikes, I rather have the parts original or close to it. I enjoy the look and feel of those friction and early indexed components. Plus, their simplicity allow me to work on them (assuming I can bend a new straight pin on this #$%^@$# spring!). If a part breaks on a ride rendering me stuck on the side of the road, it's not a disaster since my riding buddies will help me out or the wife is a phone call away.

However, if the pin on your RD has already started to bend away from the original 90 degree orientation from normal use, then I would think its days are numbered. It doesn't mean that you can't have a "hack" ready in the saddle bag to hold the RD in a reasonable gear to get you home if the spring does give way.

However, with that said, if I was depending upon a bike to commute to work, I might save the vintage bikes and components for the weekend and have a newer commuter that was dependable and reliable. I use to drive an old BMW to work which was fun until a fuel pump or alternator brush would wear out half way home. Now I'm driving a Toyota Highlander which doesn't have the same fun factor (if any) of the BMW, but dang having the piece of mind knowing that it's going to get me 40 miles down the road and back without worrying is priceless.