Hi Just Right,
I don’t pay much attention to warranties (see post #174 here) but the warranty would need to protect them against someone who didn’t maintain it and used the mattress on one side only which would wear it out much faster. If you maintain your rotation and flipping schedule it will almost certainly last longer than the warranty (unlike most major manufacturers where the warranty is more of a marketing tool).
I don’t know of any way to measure value on an “objective scale” (value is based on the parts of each person’s personal value equation that is most important to them) but as I’ve mentioned and based on the mattress construction, design, and materials and your experiences there and the service you received I would say it was good value.
You can see in this article that the top 15 manufacturers control 87.3% of the market and most of the best value is inside the remaining 13% which includes the smaller manufacturers who haven’t forgotten how to make good quality two sided mattresses that will last like yours. This is the “range” where your mattress fits.
You can see a comment at the end of post #2 here about a similar mattress made by Royal Bedding which he estimates will last in the range of 10 - 12 years on average and with your lighter body weight it could possibly be longer. I have a similar innerspring/polyfoam mattress that is over 15 years old made by a different manufacturer that was carefully maintained and still has an even surface (which of course has compressed somewhat) and is still being used today with a topper (it needed some extra softness for the 200 lb 15 year old that is using it).
Your mattress will probably last you longer than you seem to think it will and while I agree with your comments about “buyer beware squared” as far as the mainstream industry and major manufacturers … I think you made a good choice which would probably be better described as “buyer has every reason to be happy with their choice” 
Phoenix