Popularity has nothing to do with anything. TPC makes their decisions well in advance based on what they expect to be popular. And belonging to Ash is an automatic popularity boost.
Popularity for any Gen 8 Pokemon should be taken with a grain of salt because the generation just started and...
Most likely the plans came about when the premise for Journeys was decided. It was decided that Ash would travel the entire world and compete in the World Championships, and they felt that having him win in Alola would be the best way to transition into that.
Koharu is not a main character, so she probably won't get anything too major
And I don't see Go's goal changing; the idea seems to be have one protagonist focused on battling (Ash) and one focused on catching (Go)