This doesn't explain the dramatic decrease in the civic comprehension of Americans in the last fifteen years. There was a time, I think, that Trump's lying would have been disqualifying (along with many other issues with his conduct). There was a time when Americans could keep two thoughts in their head at once. Heck, people do that in non-political settings all the time, and then act like fucking morons when it comes time to ponder the future of the country.
So what changed? Why does the electorate suck so much more now? I think it points to the centrality of the conspiratorial element -- which you expressly point to, and I would argue that it should be even more centered in our public discourse. The wide availability and general acceptance of a conspiratorial mindset is both such an important part of what ails us, and what connects our experience to the rest of the world, where right-wingers are on the rise. And that's because social media puts out an infinite quantity of that bullshit.
I think what happened ~15 years ago is the emergence of the Tea Party wing of the Republican Party/conservative ecosphere. Or, as I think of it, "The Rise of the Idiots".
We tend to think of conservative media telling conservative listeners what to do and the listeners doing so, and that happens a lot, but it's not fully accurate. If something rises to a position of prominence within the conservative movement, conservative media (and the power brokers behind it) will certainly accept and co-opt whatever the niche movement is. (They did this with Trump's rise within the primaries in 2015.) And so when the Tea Party emerged and took shape out of the 2008 election, conservative media jumped right in to support it. However, the Tea Party movement was already well versed in creating an "alternative reality" to shape its beliefs as many of the adherents had long ago openly rejected provable reality as a basis for belief. And so as the Tea Party gained traction, along with it came a more broad-based rejection of facts and data in favor of "truthiness" and "ideas that feel right" as major foundations of conservative thought.
Of course, you can't ignore the rise of social media (namely Facebook) in that same period and the introduction of algorithms within a couple of years. If the Tea Party was lighted match of non-factual discourse, Facebook became an enormous gas tank just waiting to explode. With the rise of Facebook (and later other social media sites), it wasn't long before non-factual discourse took over the Republican Party to the extent that the legacy conservative media (Fox News, Newsmax, Rush, etc) were forced to adapt this mode of belief. Of course, it wasn't as if the majority had to be forced into it kicking and screaming, but it was a significant adaptation when conservative media learned that the more they lied, the better their numbers and the more they were believed.
So, to answer your question, I think the major change ~15 years ago was the rise of the Tea Party as a first wave of the "stupid Republicans" wresting control from the party leadership and what we've had since then has been a continuation and completion of that effort.