Can't remember where I read this but IIRC it's not a healthy for a machine to constantly run at 100%, hence you make an engine with a peak performance of 200mph which constantly runs at a cosy speed of 100mph.
Though for a diesel engine, at least, it's also not healthy always to run at 50% - there's been a significant problem in recent years with boat-owners on UK waterways over-specifying the engine they need for low-speed cruising, and suffering glazed bores as a result. http://coxeng.co.uk/engine/bore-glazing-and-polishing-in-die...
Fossil cars are also more efficient at lower RPMs. Designing the gears such that they can cruise at highway speeds with low RPMs incidentally allows the cars to go way past speed limits as well, if they have enough extra power.
Well, that depends on the exact model/make. In Germany there's the term "vollgasfest" (full throttle proof) for cars and it means ability to e.g. empty the gas tank in the pedal-down position in one go without damage. Especially some of the foreign brands from the 70'/80's are known to overheat in such conditions and also some of the earlier turbos. Wear is of course increased under more load but i wouldn't regard it unhealthy for engine/transmission and co per se.