dougmccoy,
While your premise is theoretically sound, there are two exceptions to it.
First, the amount of overdriving typical in the market is a factor of three with 5mm. For on topic matters, you are correct because most Luxeons are being overdriven a fraction of their rated power, but in general, 5mm LEDs are typically overdriven by a multiple.
Second, because of the above, the lifespan changes from rated life to mean time between failure (MTBF). The difference is that MTBF means that most products will last the given span, some will last longer, and some will fail the next time they are used. The MTBF of a multiple overdriven LED is closer to 1,000 hours, 20% of the life you claim. Worse than that is the problem that the MTBF is predicated on optimum conditions, something rarely found in survival situations.
I get steamed because I see overdriven LEDs being marketed as having 100K hours of life with the implied promise that they will not fail when they are designed specifically to do so. Since these products have huge numbers, that means they are failing on a regular basis. That not only makes the LED lighting industry look bad, it puts us at risk of serious liability from rapacious tort lawyers. Although it is unlikely, this industry could be strangled in the crib by frivolous lawsuits. If somebody dies and they happen to have an overdriven Luxeon as part of their death, a tort lawyer could sue Lumileds for millions and trash the industry. Think it couldn't happen? As a developer of commercial products, I'm not ready to take that leap of faith.