I absolutely think it's fair to question the validity of most reports in the general media. They often lack depth, detail, and are written by non-experts and presented in a way that can be heavily biased either intentionally or through ineptitude.
However, I think there's a significant difference between assuming that a story in the general media is lacking info or presented with bias, and assuming an actual study from experts in a field based on lots of data is incorrect because we think that science or all of acedemia has been "corrupted by liberal groupthink". Just because we can't trust the media to present information without bias doesn't meant that we can't trust scientists, or doctors, or engineers, or other highly educated experts in their fields with data on their side. I don't go to politicians to tell me the truth, or to even use the information presented by the scientific community in a decent manner. I go to the people that spend their lives studying this stuff and back it up with data. They're not infallible, but they often have actual evidence to support their position.
In this case, my family history and lived experience align with what so many experts are saying. Same is true for many of my peers, so it's easy for me to believe there's some merit to what they're presenting. Things have changed, but not necessarily gotten better for tons of Americans in the last 50 years or so. Yeah, we have more stuff, but there's less value. We've been made more comfortable, but we have to work harder and make smarter choices to end up in similar places as our parents/grandparents. There seems to be a lot less margin for error these days. Massive debt has never been easier to acquire. Unexpected medical costs can be crippling for even the most conservative people. Again, a lot of this comes down to how you judge "Quality of Life" and what role "Social Mobility/The American Dream" plays into that judgement. It seems like you're suggesting that Quality of Life is mostly up, and I could even agree with that depending on the specifics of the argument. But at the same time I'd argue that the core of "The American Dream" and "Social Mobility" are both in decline. Obama was elected in 2008 by selling "Hope" for a better future. Trump was elected in 2016 with the slogan "Make America Great Again", indicating that it wasn't very good in 2016. Both parties have recognized that voters aren't happy and tried to capitalize on that and in doing so, they've both tacitly acknowledged that things aren't getting any easier for normal people.
I think if you asked most people "When was the American Dream in it's hey day?" they'd respond "the 1950-60s". That aligns with my family history that I already shared. It was a truly unique time in history. The US was the only economic super power on the planet, and all of Europe, Russia, and Japan were rebuilding after WW2 using American made goods. The country was flooded with money from across the globe, and the Middle Class grew out of that. Wages were high due to increased union representation. Taxes were high, particularly on higher earners. Corporations hadn't all adopted the Chicago School of Economics approach and often had a mindset geared more toward being productive parts of society than singularly focused on growing share holder returns. They offered pensions to support their workforce after honest careers. I can support this outlook if needed, but didn't want to bore you by posting charts and graphs as I think it's pretty well understood and accepted.
Without massive death from another world war, we probably can't do anything to make the US the only standing economic super power anymore. We can change tax code, and corporate outlook and demand higher wages. All of those options would cost those at the top money, and all would be welcome in my book. Most studies that I've seen indicate the buying power of the Average American peaked about 50 years ago in the early 1970s. In the intervening years, the wealthiest people in our society have seen massive investment gains thanks to changed corporate ideologies (have to acknowledge that as an investor I've seen benefits from this as well), while simultaneously seeing their tax burdens shrink. I don't know that they've necessarily rigged the game, but it sure seems like it's worked out suspiciously well in their favor over the last 50 years or so, even through some massive financial bubbles and economic struggles. I don't feel the same is true for most normal people. They might have more stuff, but it's all debt fueled and built on the backs of low earners here and abroad.