I'm not sure what you are saying here. The writers didn't replace any of the other trials. We saw the Brooklet Hill Trial in SM33 (Lana may have been the one to complete it instead of Ash, but we still saw it), and we are going to see the Lush Jungle Trial (which it seems Ash is going to complete) in SM35. After SM35, we will have seen all of the Akala Island Trials except for the Wela Volcano Trial. Since the Trials are akin to Gyms in SM, I still believe we are going to see the Wela Volcano Trial and Totem Salazzle at some point. As of right now, it seems that is going to be after Olivia's Grand Trial in SM36.
I wouldn't really define that as a trial since it wasn't a task that a trainer had to do in order to progress in the island challenge.
Ash and Lana just randomly decided to fish somewhere and a Wishiwashi happened to appear there.
IMO, it was more of a nod to the games than a trial per se.