Actually I think it was Roger Rabbit. When that movie came out, the Disney animation studio decided to put new focus and energy in animation, especially with shows like Gummi Bears, Chip N Dale's Rescue Rangers, etc.
I see your point and your right that "Who Framed Roger Rabbit" showed Disney that quality animation can still be entertaining and, more important, make money. Roger Rabbit was done with Steven Spielberg and isn't a solely Disney movie. At the time Disney wasn't doing great bussiness and was being out done by Don Bluth productions esp when they release the monster success of "The Land Before Time."
The Little Mermaid is the first Sole Disney flim in sometime to do gangbuster numbers at the box-office and at the time broke the record for highest grossing animation movie, which before mermaid was held by Don Bluth's Land Before Time.
So while your right to a point, and no one can say that Roger Rabbit is an important movie in the history of Dinsey animation, the start of the era known as the
Disney Renaissance must be credited to Mermaid.