Ok - I see the point. But 
lets have a look a my situation.
Swim spa instead of a hot tub - double/triple price for a cover because of the size - will be about $1000 or even more.
Will be using it once a week(cottage) so the cover will be opened/closed rare - should not be a lot of stress at the seams ...
If I put a thermal blanket under and extra cover on top of a regular cover I hope the main cover might last 10+ years
I think that 10 years is a good number but replacing this expensive cover every 3-4 years sounds like a wrong way to go.
With respect - and I mean this to inform and hope it doesn't come off wrong - I still think you're looking at this the wrong way. The cover life is almost always directly attributed to the quality of care of the cover. If you leave the cover off for 15-30 mins after putting in chemicals, use the proper chemicals (no Tri-Chlor tabs, for instance), clean and protect with (say) 303 Potectant regularly, and dont have sun up to sun down direct heat on it, that is your best chance of having a long life of a standard cover. That is way more important than (say) adding an extra thermal wrap, larger wedges, etc.
If you're only getting 3 years or so out of your cover, you're either not doing something right, or buying aggressively-poor quality covers. 5 years is a nice average, and most of our clientele exceed that lifespan.
Consider - Lets use rough numbers: A new standard cover costs $550 (totally spitballing here but that is a rough average where I am). You burn through 3 covers in 15 years. That is $1650. Are hard-top covers costing that cheap? I know bigger covers with special wrapping do not have a longer warranty so I simply ask - what does the math look like?
I might be on an island on this one so I apologize if I am not understanding you. But I think that you're making this more complex than necessary, and only looking at it from an extreme worst case scenario.
This is a cost of doing business. If you already have a tub, I can understand on wanting to extend or upgrade. I hope it isn't something that is keeping you from a new tub purchase.
