Not breaking binary compatibility is a pure good. The standard could pull unique_ptr in as a built-in feature with a different name, and retain backward compat

Not breaking binary compatibility is a pure good. The standard could pull unique_ptr in as a built-in feature with a different name, and retain backward compatibility at cost of an unfortunate redundancy. Code using unique_ptr would gradually transition to the new thing. Of course each would have a move constructor from the other, to ease the transition.