Part of the problem is that Democrats' coalition in the past few years has been really inefficient. Hillary got California to swing left by 7 points, and Biden got similar swings from Maryland and Massachusetts. That does jack for them in the Electoral College and Senate. And while Texas may be getting closer, it still has a ways to go before it actually flips.
Their Senate majority in the 113th Congress included Arkansas, Indiana, Louisiana, Missouri, North Dakota and South Dakota. All of these states used to regularly elect Democratic Senators, but are now pretty much out of reach. They need to figure out why that is. And of course, they've continued to fall short in Florida and North Carolina. These states are only light red. If they can hold onto a Senate seat in West Virginia, arguably the Trumpiest state in the country, there's no excuse for losing one in Florida in the same cycle.
They have the same issues in states. In Wisconsin, Democrats are almost entirely packed into Milwaukee and Madison. Maybe I'm wrong, but in a fair map, I feel like it's just not really possible to draw a third Democratic-leaning congressional district.
I agree with your premise but people were making the same arguments about Arizona and Georgia last time. Some of the trends are slow but they are real. The precarious situation Republicans are in is that once these states flip they're probably gone for a long time (like Virginia and Colorado). So you essentially replace two conservative senators with two polar opposite liberal ones. It's not like a typical battleground state situation where it's going to be a centrist on either side.
The efficiency of the GOP vote has bought them some more time but it's hardly sustainable.
The other issue with losing the popular vote massively is that there are a lot of wealthy donors in states like NY/CA/MD/IL/MA/WA etc. etc. It wasn't a fluke or an anti-Trump backlash that caused Dems to outraise the GOP by wild sums. There are more Dems nationwide and they represent wealthier states with more disposable income. This counteracts some of the other GOP advantages.
