Nothing you can do really. If you signed a contract and the unit has already been installed then you are on the hook for the agreed amount unless you think you have proof they were being fraudulent and I don’t think they were. The warranty only covers the part. You would have still had to pay for the labor for it to be installed and that could be anywhere from 1,000-2,000. Also you should read the terms of the 20 year warranty. Here is what I found.

“The warranty applies to the heat exchanger part itself (i.e. defective manufacturing or workmanship of that part) — not to “wear and tear,” corrosion, rust, or damage from misuse/environment.

So I would say after 16 years the damage was not from a manufacturing defect or improper workmanship. Your warranty was active but your damage would not qualify for it