If I was a language maintainer, my problem with such proposals would be that introducing syntax is a slippery slope that might never end.
So you would like to have a map inserting syntax. Cool. That’s reasonable, and I found myself wanting that in the past as well. But then somebody else comes along and says “Hey, what about a new syntax as a shortcut for Map.put_new and Map.put_new_lazy as well? I use these every day in my work!”. And you will find yourself in the very uncomfortable situation of having to explain why introducing new syntax X is reasonable but syntax Y is not.
(And let’s not even mention the maintenance burden of every new syntax, which is a very long topic by itself, but suffice to say they carry non-trivial opportunity costs.)
I agree with you that Elixir’s current choices and their tradeoffs might seem inconsistent for a newcomer or an outsider but I believe I speak for many people when I say that I’d still prefer knowing those choices and tradeoffs – and them being a reasonably low number – than to have more and more syntax that absolutely will confuse everyone but the biggest syntax advocates (and I’d argue they are likely below 1% of the language’s user base).
Elixir has quirks. All languages do. ¯_(ツ)_/¯ Elixir’s are not many though and are thus graspable and can easily be worked around.
