How can a WHOLE air conditioner system go out at exactly 24000 miles??

What is your question?

Air conditioner worked fine when I first bought the car, took it to the dealership to get an oil change at 24,000 miles and my AC failed as I pulled in, asked them to run a check on my AC system and was told there were no leaks, but unfortunately a mold issue had clogged my compressor/condenser and the whole AC system needed replaced at a price of $5,000. Which is half of what I paid for the car. I told them my warranty had just run out not even 2 weeks prior to me taking it in. They would not accept the warranty. They said they could give me a discount of 20% off, which took the price down to $4000. I cried terribly, this is my only vehicle, I don't drive it very much as you can see it's got very low mileage. Can't afford the price of a new AC system and think it was totally unfair to bypass the warranty when several times before that I had brought it in to have the AC looked at. The only thing they could do was not charge me the diagnostic fee. The AC works, I've showed them, but only on the defrost setting. So I know it's not the whole system. But I was told not to take it anywhere else, only the dealership. Confused and upset.