That logic falls apart if you treat them as new Pokemon. Game Freak has made a concerted effort to maintain trading continuity across their newest games via Pokebank. If Megas are truly treated the same as the others, then there is no reason they would prevent the players from accessing them in Sun and Moon. They also went out of their way to place Mega stones in AS/OR, so I'm highly skeptical of this reasoning.
I don't get what you mean. We can be certain that Alola won't contain every single Pokemon in existence minus legendaries and starters. This won't prevent players from getting access to said Pokemon through Pokebank. Mega Evolutions are different though in that they require a mega stone and items cannot be transferred through bank. So they can simply decide to focus on the new features and hold off on megas until the next game which would be able to then advertise itself better by providing access to something previously available, like FRLG back then.
Yes it would be a questionable choice but so was skipping Pokemon Z for a new generation. I'm not saying this is likely or intelligent just a possibility with a precedent given the lack of information about megas and the fact that the new bracelet doesn't have a clear spot for a key stone. Personally, I'm all for megas and new megas even, as stated in my last post.