I don't really know why they've gone to single digits - you'd have to ask Boeing directly. However, the advent of the 787 disposed of the hundreds and went to single digits. You have quoted what they used to do. The nomenclature has changed.
So it is the 787-8
787-9
(787-10 if launched)
747-8.
It'll only take you 2 seconds to look on the Boeing website and you'll see what I mean. I do believe that Boeing knows what their own products are called, don't you agree?
So, no 747-800.
Only 747-8.