It is planned obsolescence, but not because of the usage of liquid metal thermal paste. Liquid metal is not only more expensive to buy but also to apply (it is very difficult to apply liquid metal thermal paste correctly, and also they need to take care to not spill liquid metal in exposed parts of the processor or it may short circuit the CPU; that is probably also the reason of the white plastic around the CPU). Also, liquid metal only form alloys with some types some metal and probably Asus knows this and avoided using them.

But it is still planned obsolescence because like most notebooks those parts like fans are not meant to be replaced, at all. They probably can be replaced, but is still expensive so most people will prefer to buy another notebook rather than replacing a fan or battery.