Much as I reckon that at the top level of any sport you should be able to arrive with the best quality brand new components available to you I can see the thinking behind it. If they want to decrease the cost I would vote for cheeper components or some shared components rather than extended lifespan.

From a purely mathematical point of view though the engine would have to last the 13 rallies I am guessing with about 250KM of closed course race speed running = 3250km + perhaps twice that on transport stages.

Baring engine destruction in a crash ( and I would think that the failure to finish as they did in F1 on the 2 consecutive races rule would be penalty enough ) I can't see that it is not technically achievable and probably a more interesting way of limiting engine performance than limiting boost or adding restrictor plates.

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Stoner & Webber 2011 World Champions