I have had some horror stories myself. In a previous house the switch in my Vaillant boiler failed, and due to a series of unfortunate events that were not entirely the service guy's fault, mixed with a bit of inexperience, he managed to total my boiler in what should have been a £100 repair. I was left with a boiler that needed replacement (in their opinion), a £600 pound bill, and my family and I were freezing as it was in the middle of winter. It is a bit of a long story, and I think I know exactly what went wrong, I can tell you about it if you are interested.
Having said that I have also had some very good experiences.
In today's world labor is expensive and appliances are getting cheaper but much more complicated, and don't last anywhere near as long. This means the pool of people that can economically troubleshoot and find the exact part that has failed and replace it is becoming smaller over time. Very few boilers made today are going to be economical to repair when they are 21 years old.
So I don't think that installers are deliberately trying to rip people off by pushing new boilers, they are just being realistic, and in many cases are actually trying to save people money. If it is a tricky problem that takes a long time to diagnose, on a boiler that may not have parts available, and that has a combustion chamber, flue and heat exchanger that may burn through tomorrow, then it is likely more economical just to replace it.
