It was pretty short-sighted of Gene to not have a cast member be chief engineer from the beginning. Did he seriously think it was a good idea to make it a revolving door?
IMO, he wanted TNG to be different from TOS. Data was really the immediate go-to guy for questions and reports about the engines, so the need for a permanent character to be the actual chief engineer wasn't all that great in the beginning (heck, TNG wouldn't even have had an engineering set at all if Roddenberry hadn't quickly wrote in a scene there in the pilot--the show wouldn't have had the money to build the set later in the first season).
"In-universe"--and in hindsight--it could be a case that during the first year of the Enterprise
's operation, several chief engineers trained on the vessel and familiarized themselves with her then many new systems before going on to other ships.