They may be dealing with a lower number of guests with the reservation system and most guests will want the 10 or More hour option to feel they are getting the most out of the cost.
AH events allow Disney to maximize turnstile counts but also giving a good percentage of guests a preferred customized experience.
And I doubt they will limit it to AH only going forward. The return of full events like MVMCP seems likely. Villians, Star Wars, Dapper nights, Concert nights, NYE etc. No end to the possibilities.
I find Epcot potentially one of the most intriguing. 10 P.M. to 1a.m. ?
The barges and SSE lighting can run without fireworks. Just speculation.
After Hours at Epcot isn't that intriguing until they put in more attractions -- When the Rat and GOTG opens, then yes.
If they really keep closing Animal Kingdom at 7 pm, then AH-DAK from 7-10 could be a huge draw, as the only way to experience Pandora at night.