Well no, because that's also a very clearly different situation and advertised as such. They clearly mean for you to get Diancie from an event and get it's mega stone in ORAS from some event guy, just like how Shaymin got you a Gracidea in Platinum. We have precedent in that every Mega Evolution advertised for XY had the base form available in XY in some way, shape or form, aside from Blaziken who was, again, clearly advertised as an event. Slowpoke is NOTHING like an event Legendary and is very clearly just another 'normal' Pokémon and situation like any other Mega. Unlike Diancie, Blaziken and possibly Metagross, it is not an event, it is not advertised as having anything special about it that's different from any other Mega you can get in the game and I see absolutely no reason for them to not put Slowpoke in the aftergame, why is this argument even a thing.
Who said this was an argument? That's a bit strong.
Discussion seems to fit better.
I'll give you the Diancie bit. I was not around for the 4th gen.
Going off of what you said, it's
because the Slowpoke line isn't a legendary, starter, or event that makes this situation unique. GF doesn't have to make the Slowpoke line available in ORAS - it's a common line just like any Caterpie, Aron, or Swinub. By
common, I mean any 'mon that is in some way available in the wild, breedable, and is not unique/one-of-a-kind - so legendaries and starters are not common. Not all 'mons are available in-game, and must be traded or transferred over. Just because one common 'mon gets a new form, doesn't necessarily warrant its inclusion in the
remake of a game wherein it was not available. Starters and legendaries? Yes. Common 'mons? No.
As for the availability of the Mega stone inheritor? GF has expressed desire to use the social functions of the 3DS this generation. It wouldn't be too hard at all to effectively utilize the GTS and acquire a Slowpoke for your new OR or AS game. Of course, one would have to put aside the blatant misuse and abuse of the GTS... Then there's also the Poke Bank - the service that will be used for years (and games) to come. In fact, I wouldn't put it past GF to do just this.
So, you want to try out your new Mega Slowbro? Well, looks like you'll just have to trade for one with your friends or pull one from the Bank. It's not hard this generation to acquire such a common 'mon in order to use its new form. Now, that's not to say that the Slowpoke line won't be available in ORAS during or post-game. It's simply a new opportunity.
Besides, we have basically nothing to go off on and we're basically speculating now and, more importantly, we have no idea what GF have planned.
Speaking of what GF have planned, I really would prefer if the contests were more like RSE contests, but brought back those seals from DP. Those were really cool and unique, I thought. The DP contests were disappointing, but I honestly never
really played 4th gen. The little I did play (god, were battles s l o w) was ok, but those seals were a very nice touch - so much possible creativity.