if the smoking analogy didn't help, think about an incandescent bulb built to last 10.000h @ 110V. You can operate it at 140V, even at 220V. And of course it doesn't matter much as long as you switch it on only 5s a day. But it's all up to your usage pattern, your kids/software (and the mistakes they commit that might leave the light on errr the CPU at 100% load and thus at max clock without you even noticing), and your luck with this particular bulb how long it lasts (in NY there's a bulb in a fire house that shines since some 100 years and never gets switched off or needed any replacement, maybe your particular CPU is the same miracle)
One thing's for sure: at doubled load neither the bulb nor the CPU will live for even 50% of their regular active lifespan, it's more like each 10% increase in clock speed halve the expected lifespan.
Let's do the math:
500 + 2*10% =~600; 50.000 / 2 / 2 = 12.500 (datasheet says 10.000)
600 + 3*10% =~800 10.000 / 2 / 2 / 2 = 1250h
800 + 2*10% =~1000 1250 / 2 / 2 =~300h

I'm actually suspecting the real figures are worse

/j