> First, if pickle a namedtuple, it can't be unpickled in previous versions.

True, but I don't think Python goes as far as to promise that objects pickled in one version can be unpickled in previous versions.

> Second, namedtuple is slower and larger than tuple, so it shouldn't be used in memory or performance critical code.

True, but I doubt that such tuples are often used extensively in performance-critical areas of code. Also, code which does care about this could immediately convert these namedtuples into plain tuples, which would be backwards compatible.