I love the fact that you put an accountant on this. I think I may buy the eight years of annual service updates, as the price may increase and I just don't want to think about that. Still, as you point out above, if the price stays the same inflation itself almost eats up the savings of paying ahead for those years 5-8. Perhaps, though, the numbers fall apart when I see that I am likely to be driving more like 18-20K miles/year, which means I'll eat up those eight "annual" service updates in just 5 to 5 1/2 years, which means I'd need to make more than 4.3% interest on that second "four years" of service to break even.
So 8 years of Service for me. I think. The EXTENDED Service agreement, which is really an extended warranty, is a harder decision. Assuming the cost will remain the same - big assumption - then waiting to buy that until just over 30 days prior to 50K miles/4 Years makes sense. If we find that the price goes up to say 4K for this extension, we may regret waiting. Dunno. Thought welcome.