They were definitely tossing ideas around for this since at least 2015/16, but it wasn't being put into active consideration until they started tearing up Critter Country like you said. I think the merchandising push was to commemorate the ride's 30th anniversary at Disneyland and they had an excuse to start pumping things out for it.
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I mean, that's when we were getting cool stuff like this and the plushies people scalped the hell out of.
Honestly I'm very surprised Disney never simply branded the Br'ers as being "Park Originals" like Haunted Mansion or Figment. Not at all surprised that they're being retired because the company wants to replace them with more "financially successful" IP instead of facing their problematic origins head on. They kept kicking the can down the road until they had a reason to announce the re-theme. It was announced way too early (when only one piece of concept art was publicly released!) and had they waited now instead of mentioning it back in 2020, we'd likely have less of a polarized reception to it. I'd like to keep an open mind, but the two year wait and lack of substantial updates makes me feel like it's been overhyped and what we'll get will satisfy no one at all. Not the people in support, not the #SaveSplashMountain crowd, not the guests, no one.