It is easy to add features to a language. It takes courage to take features away. If you just add every feature than anyone might want, then you end up with something like Perl. Which incidentally seem to be much more popular than Python, so maybe Larry Wall have a point.

By "dumbing down" I assume you mean "looks less like lisp". You can do the same as map and filter in a more elegant and pythonic way using generator expressions.

Btw. he was just thinking aloud. In the end he just moved map and filter out of the core language and into a library, and lambda stayed.