At the release of Sideways, everyone and their brother knew the major wine varieties, and had opinions. Indeed, a major reason the merlot spiel had such an impact with so many is precisely because it was so popular. Many like to be a contrarian, so if something knocks on the popular thing that can have an impact ("who else can't stand Tiktok, right?"). Most of the varieties of merlot were very high quality, and I doubt the overwhelming bulk of wine drinkers could taste any deficiency (many can't even differentiate varieties).
https://www.guildsomm.com/public_content/features/articles/b...
That's a much better article about the rise and fall of California merlot, and basically notes that merlot had plateaued by Sideways release, and there's a bit of a cause/correlation mixup happening.