How did you pay for the tickets? I am guessing miles, but please correct me if I am wrong.

If you paid with miles, I would say 20k is reasonable, but on the low side. F is 20k more miles than Y, so AA just refunded the difference between Y and F. I agree that the recline is a big part of what makes F so good, so it is reasonable for them to refund the full difference between Y and F. They should consider giving more miles as a gesture, IMO.

If you actually paid $8k, I think that changes things, but I suspect you would have said 'cost $8k' and not 'valued at $8k'.