Lean does not make more power. Only rich does. The reason running lean is destructive is because there is oxygen still present in the mixture. This enables metals like a hot aluminum piston to oxidize and burn. It is a lot harder for a piston to melt by sheer heat and change its phase of matter without the presence of oxygen.
The combustion process is also actually slower when lean. This is proven if you've ever had a backfire in the intake. The fuel was still burning at the end of the exhaust stroke and the new intake charge ignited. What makes power is to have the quickest burning mixture and to have the highest amount of cylinder pressure around 5-10 degrees after tdc. This is the primary reason spark is advanced as rpm increases. The burn rate of the mix remains the same but in order to get max pressure just after tdc we have to lead the spark and begin combustion a little bit sooner.