8 years and how many miles? I think 20% is a pretty good offer. I would take it.
I'm surprised they offered you anything. Trying collecting on the life time warranty you get with a roofing company.
Life time warranties are loaded with checks and balances that discount daily use. Maybe you can post the warranty words for us to read?

I think their warrenty may include. Damage resulting from normal wear and tear, including the results of fatigue, is not covered. Fatigue damage is a symptom of the frame being worn out through normal use. It is one kind of normal wear and tear, and it is the owner's responsibility to inspect his/her bicycle.