tl;dr: Magic and Psychic are concepts that overlap too much, so I don't see it warranting existence.
*looks at Rock-type and Ground-type*
You hear that, guys? One of you shouldn't exist.
I have also seen a lot of the "it's too late to add a new type" argument. I have seen this very argument posed since I initially joined SPPf back in 2006 before D/P were released. I think "late" is a very relative term. Say they introduce a new type in Gen 6. 10 years from now, when we hit Gen 10 or something crazy and have 1,500 Pokemon, people aren't going to look back and think "man, that new type was introduced way too late."