I said this about Zygarde 100 & this still applies for Megas. The Pokemon Company does not market something like that to consumers, succeeds in a huge way, & remove/not do something with it.
Charizard, Mewtwo, Venusaur, & Blastoise lines are still the most recognizable & popular Pokemon next to Pikachu. They won't remove something that enhanced those characters.
This 5th move thing is probably limited to a select few Pokemon too. This may be their way of boosting Pikachu.
So it makes no sense that they dropped precisely these Pokemon in the first games of Generation 3 4 and 5, right? True, you could just transfer them from past games in generation 4 and 5, but what about gen 3?
Megas are new Pokemon. They might not have a pokedex entry and be temporary but they can't be scrapped altogether just like Raichu can't be erased regardless of how much Game Freak regrets introducing it. But that doesn't mean there's no chance they won't be in Sun and Moon. Game freak might opt to remove mega stones ( or at least some of them) until the second installment of the generation comes along, just like many existing Mons won't be available naturally in Alola. This would make more sense if 4th gen remakes are on their way, especially since remakes need to revamp and bring back old stuff in order to advertise themselves ( something like: hey players, missing mega Garchomp? - not really - well, guess who got one with a powered up Synchro move?)
But that's speculating wildly. The point I'm trying to get across is that it's not impossible for us to have no access to mega stones in Sun and Moon if they're trying to focus on the new features, even though I'm positive that their data will be in the game, as will the data of Pokemon that are not naturally found in Alola. I'm not saying this is likely since it would be a questionable move and we have already seen plenty of mons that can mega evolve in the trailers.
On a personal note, I'd much rather see new Mega Evolutions than new Pokemons especially when they look as... weird as drampa or bruxish or the new panda. We know most of them will end up being low tier and largely irrelevant competitively, so why keep filling the Pokedex with mons that'll be forgettable a few months after release when you can make something nostalgic that at worst has some potential? Good job on the abilities though, hopefully they'll get good distribution and not be stuck with some random gen 7 Mons. Sadly this seems unlikely, given that most pokemon already have 2 abilities and a hidden ability. A couple might get retconned, but I don't expect much else.