I understand the view from the poster, most basic list operations are using brackets, ie reading and writing with [], delete with del L[], why not append ?
And being used extensively, that brackets are annoying.
And yes, += [] is more "concise" than .append() so some people would think it's more clear because "it's smaller" (they'd be wrong as the OP mentioned).
But it would break the "There is only one Obvious way to do it" principle.
People would take time to figure out "okay, what should I write ? What's the most Pythonic ?"
If someone wants to use their own list class, it's doable with the current syntax :