If the new 12V-2x6 connector was incompatible with the old 12VHPWR connector, they should have (and would have) made it physically incompatible. They didn’t. You cannot blame the user for doing something which is specifically allowed by design.
We really saying this when swapping a PSU to a different PSU and reusing existing cables that all look and plug in the same fries your build.
I think it's utterly absurd that this is the case but that's PC components for you.