I don't know if that's true. Nothing is ever static. For one thing, Trump has been hemorrhaging support from Latinos. Every election is a new electorate.
Yes, the situation will eventually change. Trump himself does seem to be speeding the process up. But my read right now is that the situation has too much momentum for one person to change it.
Here’s why: let’s rewind back to 2008, with the financial crisis. A lot of ordinary folks lost their shirts. My wife lost her high-paying job in finance as a result, for example (she was a software developer for banks, nothing to do directly with the bad guys in the crisis). That was the last straw in a long litany of insults and injuries to the working class, that had been seeing longer hours and fewer benefits over time while during the same period their purchasing power was either staying flat or declining, all while a few people were attaining massive levels of wealth for comparatively little work.
The electorate wanted change, which is how Obama was elected. He turned out to be a centrist who protected the bankers from the consequences of the mess they caused, even when those bankers took taxpayer funds to do stock buybacks and pay themselves huge bonuses. The level of anger that was growing was obvious to anyone who cared to look. Obama was able to win reelection mainly because of the ACA and the fact that his opponent was perceived as even more of a flunkie for those same economic elite.
Trump cared to look. He saw all of this, and played to it. The voters were looking for someone to blow up the system that had been screwing them raw for fifty years. The fact that the Dems twice ran centrist business-as-usual candidates when they had a progressive option just shows that the Dems were unaware (and largely still are) of the reality. Convention wisdom would say there’s no way America would elect a professed socialist to the presidency. But conventional wisdom twice said the same about Trump. No way America would elect a p*ssy grabber. No way America would elect a con man and a felon. The reason he got elected is that folks are tired of business as usual.
The problem the Dems invoked by running Hillary and then Harris is they showed the voters they stand for business as usual. And that’s going to be a very difficult image to shake. The blow-it-up party is now Trump’s party, and it’s going to be hard to get people to listen to the counter message. Trump can certainly help that process along and he seems to do so on a regular basis, but nothing he’s done yet is so bad a screw up that voters wouldn’t vote for him again against anyone with a D after their name. It won’t be permanent, but the Dems running Bernie now would just seem like cheap imitation or insincere gimmickry. I’m afraid I don’t see an easy way out.
The problem does go even deeper: the perception is that Dems are generally milquetoast politicians who will refrain from taking bold moves when needed. The Reps do not suffer from such an image. Again, hugely problematic right now. The Republicans made it into a street fight in 1994, and since then it’s been the Dems thinking they in an adjudicated points match when in fact they’ve been in a brawl complete with knives and bats, pistols and shotguns, and they just seem unable to grasp the simple facts of the case. To change all of this will take a massive effort. The old guard will have to slither off and die, most likely, before there will be a chance for it.