(Yes, this means you, Biden and Buttigieg.)
The danger lies in the reality that capitalism is failing more and more people, and that no centrist is capable of squarely addressing this fact. The latter is for the simple reason that doing so will require a degree of confrontation with capitalists that no centrist is likely to possess the personal constitution for. Just for openers, it will endanger said centrist’s access to the campaign cash that he or she needs.
A centrist is capable of winning the election. All that takes is the correct amount of the correct sort of propaganda, and (thanks to the capitalist class) any centrist can reasonably expect to be rolling in the campaign cash necessary to procure the needed propagandists for that exercise. (It would amount to a snow job, of course, but since when have snow jobs stopped politicians from winning?)
The problems start after the hypothetical centrist Democrat wins. The economy is only going to get worse (there will be a recession; recessions always happen sooner or later). And it doesn’t even take a recession for a centrist’s tone deafness to hurt him: witness what happened to Marcon (now polling below 30% in public support) in France.
In France, that’s not necessarily a big tragedy. That nation has a multi-party political system, and parties on the left seem to be successfully capitalizing on Marcon’s deficiency of class consciousness. Then you have the energy in the streets, and a long and time-honored tradition of a populace being willing (and sufficiently organized) to exercise it.
In the USA, it’s rather different. The centrist will be president under the Democratic Party label, and will taint the rest of that party with his stench. There are no viable third parties. There is no viable radical movement with a history of semi-regularly making its presence in the streets known.
There is only a Republican Party that has discovered how compatible fascism and the bourgeois state can really be. And odds are the next fascist to lead the Republicans will be significantly more competent than the current one. (The odds have to be such, given how low on the competency curve their current standard-bearer ranks.)
And all the above is assuming the centrist will win in 2020 in the first place. That’s hardly a given, particularly if the candidate simply runs on not being Donald Trump and nothing more.
None of this is to say that a liberal or left democrat is guaranteed victory, or that the concerns of non-leftist voters can be ignored. For example, there is a real pitfall in pushing for Medicare for all in a way that immediately abolishes private insurance, because it will alienate many economically privileged voters who might otherwise vote Democratic to get Trump out of office.
It’s just that this street runs both ways: you can’t ignore the concerns of the less-affluent voters, the ones who have lost out big in recent decades, either. At least, you can’t ignore those concerns unless pandering to wealth is more important to you than preserving the future of a free society.