One day later I still think this is great business idea, that some company ought to pick up. Or, some individual.
In another thread, someone pointed out, that some organization does a 12.5 khour test. But that's unconvincing to me, since buying a lamp and let it run for close to 2 years, and then measure the light output is totally unrealistic, because it doesn't take ageing of the elements into account. I don't want a bulb that emits some 70% or 80% of the light after 2 years of continuous burning; I want a bulb that emits some 70% or 80% of the original light after I have been switching it on for 3 hours a day; after the accumulated 12.5 or 25 khour. You can't test that, you can at best artificially age the lamp.
In a nutshell, yes, I'd be prepared to pay considerable money for a LED bulb that is
1. date stamped, AND
2. has its output measured after manufactured, and documented, AND
3. where the manufacturer guarantees that this bulb will produce at least, e.g. 75% of the light after 5 years in service.
Remember those bulb testing equipment in shops, at least in Europe, where you could test your new bulb before going to the cashier? My suggestion in this respect is, to set up similar testing locations in future, where one can walk in and test the remaining light power of one's bulb(s). With a unique serial number, like stamped on the side, and the original value stored with the serial number, just bringing one's bulbs, one can easily test one bulb(s). With a reading of the serial number and the display of the remaining efficiency, everyone could check his/her bulbs. With an (almost) automatic exchange, if the performance of the bulb remains below a guaranteed efficiency.
As I said, that's what I'd call fair; and be prepared to pay extra for.