Game Freak isn't that arbitrary...
Not to mention that it would imply the weirdest take on the "patching in new Pokémon" theory people keep dragging up: ORAS will then have to transfer content to X/Y (as no more Mega Evolution models or stones are coded into X/Y than those two we know about), but without being able to make use of that content itself. Sounds reasonable.
As for why Sceptilite and Swampertite should be a thing, fanservice is as good an argument as any. The more Megas, the merrier, and it balances nicely with Blazikenite as has been mentioned countless times already. When GameFreak has pushed the entire "Hey, kids, Blaziken can
MEGA-EVOLVE and become even cooler!!" shtick in X/Y, it would be weird not to present a similar feature for the other starters lest they want them to fade into obscurity. The designers have stated repeatedly that the starters are designed for balance, and by making one an objectively better choice in-game (or by offering it a post-game upgrade, depending on when the Stone will be available) that balance will be broken badly.
There's also an argument mentioned in an obscure post somewhere in this thread: Sceptile was hit pretty hard by the Physical/Special split. It had a great Special attack in Gen. III, making good use of moves such as Thunderpunch, Dragon Claw and its signature Leaf Blade. Those are all Physical moves now, as is most of Sceptile's damage-dealing level-up moves (Leaf Storm and Absorb being the only exceptions). It's hit by a bad case of movepool/stat desynch, which a Mega Evolution could patch nicely.
On the other hand, Swampert doesn't need a Mega Evolution that badly. Okay, it mostly learns Special STAB moves upon level-up, while its Attack is Base 25 higher than its Sp.Atk, but Swampert was never much about attacking anyway. It has only one single weakness, Grass, and thus is capable of tanking most things the game throws at you (there is no Grass gym leader in Hoenn, after all). Swampert is nice defensively, but can pack a punch on the offense too (with a little investment of TMs, since its level-up movepool is linked to its weaker offensive stat). But given that Sceptile gets a MEvo, there's no reason why Swampert shouldn't. Something along the lines of Mega Venusaur, capable of tanking moves to high heaven and hit back decently hard. It would compliment the ultra-offensive Mega Blaziken and the (presumably) agile Mega Sceptile.
Also, some food for thought: Even given a level progression similar to RSE (whose level progression was a little slow, if I may say so myself, so it's likely to be adjusted upwards), players are likely to fully evolve their starter sometime before or just after the sixth Gym, in Fortree. The next Gym is in Mossdeep, where Steven Stone lives. For those who don't remember, Steven likes to collect rare stones. A visit to his house after beating Tate&Liza would be the perfect opportunity to receive the Mega Stone for one's starter Pokémon, both story-wise and game progression-wise. The Mega Starter would be ready for use in the final showdown against Team Aqua/Magma, the final Gym and later the Pokémon league.
"
But that would be overpowered!"
Well, so was Groudon/Kyogre the first time 'round. Admittedly, though, you only caught those after beating Aqua/Magma. Still, you got them before taking on Sootopolis Gym, and way before the Elite Four. Not to mention that in Kalos, you got your first Mega
just after the third Gym.
Either way, I have faith in new Megas. GameFreak set a strange presedence when they gave Blaziken a Mega Evolution, and we've seen for many years how much they like to do things in pairs and trios. They rarely do major things to one duo/trio member without including the other members in the same deal. Not since... uhh... Scyther got an evolution and Pinsir didn't?