That’s actually 16 times less resolution! the height and width are both 4 times less so you actually get 4x4=16 times less pixels!
I can’t imagine any scenario where anyone would want less than 15/16, even in Concorde at 60,000 feet or the darkstar 
The highest you can go up to is Zoom Level 23, this is because of the system that is used. (The technical reason is because the coordinates use 32 bit integers so you run out of number to do higher than 23.
I went into a lot of detail about this subject here, and this wishlist topic is for what you are both talking about/requesting:
https://forums.flightsimulator.com/t/option-for-aerials-texture-resolution-zoom-level/732517/68
and
https://forums.flightsimulator.com/t/option-for-aerials-texture-resolution-zoom-level/732517/75