I agree about the fingerprint scanner, I agree that One UI needs more refining. And about S8, all phones, irrespective of budget and premiumness, receives only 2 major updates as it is controlled by Google as of a software bond.
It's Samsung policy to provide 2 major update. Google ensures all devices must receive atleast 2 years of support from its manufacturers. Otherwise, I doubt the way Samsung handles it wouldn't have even provided 2 major updates.