Advantage Airtech Climatecare

On June 16, 2021 AC maintenance was done by Technician Rick and said AC was working well. 2 weeks later we noticed water was leaking. We called Advantage Airtech on August 16, 2021 and Rick came to do diagnostic and said drip pan was broken and endcoil rusty and needed replacement. Wanda provided us a quote of $1500.00 +HST for the part and labor and she also presented us an option of replacing the AC unit with a new one for $3800.00 +/- plus HST. Our unit is only 11 years old. So we requested someone to make an assessment of our existing unit should we need a new one. Sales advisor Darcy came and was very helpful and indicated that the coil change is more reasonable. He also suggested that Furnace maintenance service be done as soon as possible instead of waiting for having it done in the Fall to ensure there is no damage due to water leak as he saw some kind of rust inside.
On September 9, 2021, the AC evaporator coil was replaced. Furnace and Humidifier maintenance were also done. Service was provided by Matt. Matt was helpful and tried to answer some of my questions. He also indicated that the refrigerant was a little low and may have caused the build-up of ice causing the water leak. Matt seemed very thorough with his work. He explained to us many things. I would give him a 10/10 for the service he provided.
Here are some of my questions that still linger in my mind:
1) Our AC was installed in June, 2010. It was supposed to be a better unit and more expensive. How come the coil got rusted in only 11 years? What is the material used for the coil?
2) We registered for the Advantage Airtech ClimateCare Program. Maintenance of our AC, Furnace, Humidifier had been done every year. Could the AC water leak have been prevented?
Otherwise, our experience with Advantage Airtech has been satisfactory.