Is $1,000 to clean AC coils (outside heat pump and inside air handler) a fair price?

We purchased our entire AC system from McCarthy Home Services 8 years ago, and it came with a 10-year warranty. A previous tech found a slow leak, so we called them for warranty work. Now, they are saying the coils in the outside heat pump and inside air handler are dirty and should have been cleaned annually by us, the customer, and that this isn't covered under warranty. The tech claims the outside unit only has about a year left and the system is barely cooling. He suggested they buy out the remainder of our warranty and we buy a whole new system for around $15k. We paid $7800 for the current system 8 years ago, and I thought these systems were supposed to last 20 years. This doesn't sound right to me. I don't understand why coil cleaning and maintenance would be solely on the customer, especially since no one discussed a maintenance schedule or explained this when we purchased the system. The paperwork we received doesn't detail the warranty or mention maintenance. A previous tech came out twice before and never mentioned cleaning the coils, only testing pressure and noting a leak. The tech who came out recently left without doing anything after arguing, claiming the leak was due to not cleaning the coils and wasn't covered. My dad watched some YouTube videos and is going to try cleaning the outside condenser coil himself using Nu Coil and Frost King ACF19 Foam Coil Cleaner. We aren't sure how to clean the inside coil. The company quoted us $1000 to clean both, which seems very high. I'm waiting for a callback from management to discuss this. I feel they should have offered paid maintenance plans after the initial included visits. I also question why the tech didn't address the dirty coils first before diagnosing a leak. I'm worried about paying a high price to have the system re-tested after we clean the coils ourselves. I'd also like a second opinion on whether the outside unit is truly damaged to the point of needing replacement next year just from not being cleaned.