I'm pretty sure all of the new starters will get one, in order to ensure that all players have access to a Mega Pokemon in their main playthrough, with a Pokemon that has a higher-than-average chance of being a "permanent" part of their team. We've already seen Mega Blaziken, and a conspicuous glowing orb in that wierd scene from the Origin trailer seems to point towards Mega Charizard. That makes three Mega Fire starters, which would be a bit unbalanced.
Socran's Razor: "Never attribute to massive stupidity by someone else, what could be explained by a tiny bit of stupidity on your own part."
Otherwise it'd be mega-overkill.. Too many of them.
And it's not like you won't have access to a Mega-Pokemon if your starter isn't one.
I imagine Mareeps will be catchable in the wild, and Mawile, and Absol, and Riolu (probably split across the two versions) - Plus Torchic is being offered with its stone via Wi-fi Gift.. and I wouldn't rule out a Mewtwo being captureable in the post game.
So yeah... There's no need for all three starters to have a Mega Stone hold item.
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