I'm afraid "the left" hasn't had a clear meaning for many decades now.
The meaning of what is left and right shifts over time and whatever method you choose to place the middle is where biases appear.
If no party to the left of A has a chance of government and no party to the left of B has a chance of government, you've placed "middle" in the wrong place.
Ignoring political reality by starting a history lesson isn't going to create changes.
It's likely to lead to voters involuntarily disenfranchising themselves and not having any effect on the duopoly the system encourages.
The left is not relative, nor has the meaning shifted in all this time.
The left is the same communists and anarchists it has been for over a hundred years world wide now.
Political party popularity does not change political ideological meaning.
And the American system doesn’t encourage duopoly, it literally enforces it. So yes, of course many leftists are going to feel disenfranchised after close to a century of being villainised and neglected by their “representatives”. The solution to that is for a party to adopt leftist ideals, but that goes against the interests of the ruling class who’s money and influence runs the game.
The left is relative. Otherwise we still believe in the solutions of 50 years ago now.
Try nationalising manufacturing and farms. See how well that works. That was left wing once. Now it's not. Even if there are still things you would nationalise.
You're trying to create absolutes to argue easily against. That's often the way political discourse goes but it's wrong.
By all means build a straw man and totem of the left and right but it's far more interesting to find the nuance and use your intelligence rather than treating the debate like a team sport to be won and lost.
Some do. Plenty on the left don't because studies and examples since have shown where public ownership falls flat on its face and where it's the only efficient way of doing things. As well as the grey area in between.