Getting rid of the flying type would release it from stealth rock weakness and Altaria is more fairy than dragon if judged by it's looks.
Well if Mega-evo would be fairy Psychic would still be Gallade's normal primary type. Besides for example Psyduck is not psychic while it even has Psy in it's name
Mega-Golduck could have psychic-type though.
Despite all those "Fairy to Gallade doesn't make sense" comments I think it does make sense, but it is no way necessary though, but possible. It's pre-evos both have fairy typing so it could have as well in mega form. People obviously disagree about whether or not it
should have such typing. It's fine either way, megaevolutions are just extra they are not like normal evolutions but another option to use evolved pokemon. Many times pokemon may be better as ordinary non-mega with some item like leftovers or choice items. Like in case of Garchomp.
So even if Gallade would get mega-evolution with changed type (how about Psychic/Steel or Fighting/Steel?) it doesn't mean that evolution would somehow completely replace old Psychic/Fighting Gallade. I just find it interesting if new mega-evos would have unique typings and Fairy/Fighting would be very interesting type combination.