Condemning other Pros,if the shoe fits man,wear it!!,what I said;
Depending on the real issues,the OP could have a right to be upset.
Undersized ducts,reduce capacity,coupled with an oversized system,in 15 years,a lot of extra money could have been spent.
Duct leakage from a poor install would add to the operating cost.
Improperly sized line set,lost capacity and equipment damage.
And the list could go on.Just because it "worked" for 15 years doesn't mean all was or is well.
There's a lot of "working" systems out there with issues,if you can find the issues and explain it well enough to the owner,there's a lot of extra work for your company.
And it also could be nothing wrong with the system at all. You simply chose to go for the throat of the HVAC company. That is certainly your right to do so. I just prefer to not feed into the frenzy that HVAC contractors are bad people that cannot be trusted to do their job properly. But of course you don't see yourself as the problem, you see yourself as the solution; no matter who you have to tear down.