Viktor, I love your reasoned, logical and common-sense interpretation of events like these. Observe, analyze, hypothesize. You are part of that microscopically small percentage of fish "rescuers" that actually performs rescues. Most of the rescues we read about are laughable attempts at rationalizing the purchase of a fish that is in reality going from one set of horrible conditions to another that is perhaps different but often no better. A keeper sees a fish that is in bad shape but it happens to be something that he/she really wants, or perhaps a kind-hearted "rescuer" finds a fish that is clearly on its last legs and acts without thinking.
Either way, those aren't rescues; they simply add to the unfortunate victim's problems and do nothing to prevent its inevitable demise.
Your description of the cumulative stresses and indignities to which these fish are subjected, and their eventual inability to bounce back from new ones, sounds spot on.
I can't resist the temptation here to comment once again on the importance of knowledge and research into the requirements of a given species of fish prior to purchase...and then to practice some common-sense restraint when making the decision to purchase or not to purchase fish that are simply not well-suited to the water that can be made available to them.