I haven't researched it thoroughly, but I believe the use of long life plugs is driven by the requirement for cars to meet pollution standards for 100K miles. Manufacturers must certify (presumably through some type of simulation testing) their cars still meet the relevant emissions standards after 100K miles of usage.

Either it's a requirement that plugs not be replaced, or the manufacturers (reasonably, in my opinion) know they can not expect consumers to replace the plugs. So in go the platinums so the vehicle can pass emissions durability testing.

Your average consumer doesn't read the owners manual, isn't aware of when the plugs are to be changed, and won't take any action until they can tell something is wrong with the car. Fortunately newer OBDII cars have misfire detection and will light up the MIL if the plugs are too far gone.

- JimY