The main problem with how this gear ages is lack of maintenance. Professionals maintained their equipment on a regulart basis.. not wait for when something goes wrong as most amatures do. By that time is it almost too late so the wearing effects of this become very damaging.
We take for granted what we have at such bargain prices.. once these monsters denanded a pretty penny but earned more than their weight in gold... so the level of appreciation has goen down. So a CLA on that 180mm lens will cost you $90, a repalcement used one will cost you $100.. you'd go for the $100 and the old one becomes a paper weight. But new that lens would have cost you $500 or more so the CLA was well worth it.
More than likely it;s jsut a clceaning n maybe a spring slipped off it;s post.. not terrable damages you are predicting. I repair lots of these lenses n rarely do I need to replace parts... especially since you say it is still purring just not closing it;s eyes... CLA.
Anyway.. if you want I can tell you how to open her up n CLA..
or as I said I can use the parts to give another lens a new life.