That's a fair assumption that doesn't prove itself in practice. In practice, a Prius engine lasts forever. Someone will jump in to complain about some anecdotal failure but the fleet statistics are strong. They even outlast other non-hybrid Toyotas. There are a lot of reasons for this but a significant one is the lack of belt-driven accessories, lack of friction clutches, and ability to avoid high-stress operation because of the electric side of the power train.