Good point; some cars are more "failure resistant" than others. The sludge-prone Toyotas would not sludge if the oil was changed exactly in accordance with the directions, such as more frequently when encountering "severe" conditions.
When such a car is brought into the dealer with a sludged up engine because of severe conditions, and the oil has been changed accoding to the light service schedule, the garage does not know what happened. And it has to be treated as a warranty case.
Synthetic oil is not a cure for stupidity or lack of observing the directions. I have several friends who owned these "sludging" engines and drove them several hundreds of thousands of miles before disposing of them.
There are engine designs that require synthetic due to the severe operation temperatures and pressues.
Car manufacturers have to treat the customer as being negligent from day one, and design the engine to be as idiot-proof as possible. Toyota learned the hard way that practically all driving is severe, with a few execptions and that owners usually do not follow the drain interval directions.
Toyota manuals now do not differentiate between light and severe, and specify 5000 miles for all driving, knowing that most owners would pick the long interval to "save money".
The early GM V6 engines, for instance, had very small oil galleries, which plugged easily causing lack of lubricant flow. This caused many failures.