The biggest problem with this theory as posited is that the definition of left and right hasn’t changed at all in the past 40 years.
I have yet to see the goal posts moved to the right in any definable measure in the past 30 years. Less government, less government spending, lower regulation, pro-choice, support of the U.S. Constitution, individual rights, self-determination, strong national defense, supporting interstate trade and commerce without restriction.
The left has moved from equal opportunity to equal outcomes. It has moved from safety net programs to government guarantees and tried to define them as rights. It has foisted the interests of minorities ahead of equal protection to special protection and more importantly, racial and group exception from legal prosecution. It has gone beyond advocating changing sources of energy as the market dictates and allows to mandating it and funding losing effort after losing effort with great financial hardship at our expense.
I could go on and on but we know how the list goes.
The definition may not have changed, but the “progressives” are far, far more openly following the logical conclusion of their mindsets.