...Didn't think I'd have this much to say about this, but here I go, I guess.
I think what helps clear things up in terms of why
The Little Mermaid is considered the start of the Renaissance and not
Who Framed Roger Rabbit? is because
The Little Mermaid is a solo venture for Disney Animation while
Who Framed Roger Rabbit? is co-production with other studios. Additionally, while those from Disney Animation are in the credits of the film, the studio itself isn't listed in the film's end credits and the film credits Amblin Entertainment and Touchstone Pictures instead. A lot of public accounts frame the Disney Renaissance timeline consisting of films where Disney Animation is the primary studio, thus we end up dealing only with films belonging to the "Disney Animated Canon."
The Little Mermaid is something that Disney Animation can truly call its success their own and, like others have mentioned, it couldn't have been done without the huge influence that Howard Ashman (and Alan Menken to an extent) brought to the studio itself when
The Little Mermaid was in production. When glossing over the history of the Disney Animated Canon, it becomes much more apparent that the eras of Disney Animation are actually marked by big changes, influences, and strategies that happened to the studio, mostly internally.
- The Wartime Era happened because of... well... yeah. Walt Disney Productions went into debt because of it and low box office income. It's pretty much why the films we got were "package films" because they were fairly inexpensive to produce compared to full-length films.
- The Silver Era happened as the above debt started declining and Walt Disney wanted to return to full-length films to restore the studio financially, which turned out to be a huge success.
- The Bronze / "Dark Age" Era happened because of an unclear direction the studio faced after Walt passed. Though some films were moderately successful, most were box office bombs and it would be nearly 2 decades before the Disney Renaissance happened.
- The Post-Renaissance / "Experimental" Era happened as a need for the studio to create something different after audiences found later Disney Renaissance films becoming formulaic and stale.
Which brings us to... the "Disney Revival," which is notably the studio's financially strongest era yet. This era actually happens to be one of the eras that Disney itself has acknowledged that it exists alongside the Golden Age (the first era and the one before Wartime) and the Disney Renaissance.
There's been a lot of debate as to whether or not this era starts with
Bolt or
The Princess and the Frog, and... I really get why many would consider the latter to be the start; a return to 2D animation after announcing that the studio would no longer be doing 2D animated features, the studio's first full-on musical after arguably
Mulan (
Tarzan and other films featured some songs, but not a lot of characters actually singing), debuts the first new official Disney Princess also since
Mulan, the film being a notable financial success to the studio... Like, I get it.
However. The big hurdle this has is the fact that whenever Disney acknowledges the Disney Revival,
Bolt is
always included. The common argument for
Bolt is that it's the studio's first financial success since
Lilo & Stitch, and while yes that's true, I think it goes deeper than that. I think this has to do with the fact that Disney always frames the start of this era having to do with J/hn L/ss/t/r becoming Chief Creative Officer and Ed Catmull becoming President of Disney Animation and the overhaul of the studio they brought with them, including intervening in films that were already in production, namely
Meet the Robinsons and
Bolt.
Also Disney insisting that Bolt as part of this era would explain why I weirdly keep seeing Bolt merch in the Disney Parks despite never doing anything else with it otherwise.
While it could be argued that
Meet the Robinsons should be included in this era by that logic (which I've seen some do), I think the reason why Disney always choose to include
Bolt and not
Meet the Robinsons in this era is because the retool of
Bolt was really substantial and different to its previous version,
American Dog, while
Meet the Robinsons only had 60% of its film redone.
Bolt had L/ss/t/r and Catmull's hands all over it, not so much with
Meet the Robinsons.
As I've said before, it's clear that the start of eras in Disney Animation are defined by the internal change and influence the studio goes through. Disney acknowledges it here with the Disney Revival and with how
Bolt was developed, it makes sense as to why they want the era to start there. It being financially successful just so happens to be a bonus.
...Oh, you thought I was done? With L/ss/t/r gone, we now have
Frozen director and writer Jennifer Lee as the newest CCO of Disney Animation. Under her, the studio is expanding in new ways the studio hasn't before, including expanding into animated series and developing new projects featuring another return to 2D animation, as well as training a new generation of 2D animators. While I haven't seen as much people say that Disney Animation is in another new era (they do exist though, I checked), I'm inclined to believe that we are. The films under Jennifer Lee have also felt a bit more experimental than the ones before L/ss/t/r left, and I feel like that's definitely gonna be more apparent in upcoming films like
Wish as it aims to combine the traditional watercolor style used in old Disney Animation films with CGi.
The question is if a lot of people start considering this a new era... does it start with
Ralph Breaks the Internet or
Frozen II? Short answer? I'm personally going with
Frozen II since
Ralph Breaks the Internet still credits L/ss/t/r as Executive Producer.