Hi Dave, I'd imagine the issue here is that you have (sensibly) interpreted "lifetime" to mean as long as you choose to have the product, but that is rarely what they mean.
I regularly read small-print (very sad!) and I've often seen "lifetime" caveated. They most likely mean the expected lifetime of that type of product - one example that springs to mind was a Dell laptop battery. I was surprised to see that it said it was expected to last only "300 charges".
More relevantly re. solar stuff ... I'm sure when my inverter developed a relay fault - not long after the guarantee expiry of course - I was told by someone that you'd expect an 'average' (i.e. non-major brand) inverter is only expected to last about 5 years on average. Wish I'd known that before I invested so much in a PV system! It does seem rather low though. In my case, fortunately I was able to hack out the grid relays and replace them with high quality ones.