I'm not sure I would agree with this assumption. PHEVs are powered by an electric motor, with the engine acting as a generator when the battery is depleted of charge. Both the battery and electric motor are (mostly) maintenance-free, while the reduced load on the engine reduces long-term wear and tear.

It's important to note that engines go through high stress when started from cold (until the fluids heat up) and this heavy wear element is removed, with the battery and motor taking the start-up strain. I think PHEVs (which are charged regularly) will last a lot longer than an ICE car.

Add in the reduced brake wear (due to the fact PHEVs have regenerative braking that uses the kinetic energy to regenerate the battery for extra EV miles) and you have a recipe for an efficient and low-cost car. The important bit is to keep the battery charged at regular intervals, otherwise, you'll lose the fuel economy benefits.