No one questioned whether there will be more megas than revealed so far or not. I think the point
Patrick1088 brought up is whether there will be as many new megas in ORAS as in XY.
Considering 13 megas were revealed prior to XY's release and 13 megas have already been revealed 2 months before ORAS' launch (and they're guaranteed to unveil one more - and given the way they have advertised so far, they are likely to reveal more than just that one more), one is inclined to be believe we will get over 28 megas in ORAS (the number of megas in XY).
I don't know. First when they revealed the mega starters, I was happy and thought they were just adding mega Sceptile/Swampert to complete the starter trio and mega Diancie because... they wanted. Then mega Sableye showed they wanted more. And mega Metagross made it clear they wanted even more. Now we've reached the point where we know more megas for ORAS prior to release than was the case for XY - the game that introduced the concept and whose storyline was closely related to it. On the other hand, ORAS' mega unveilings began 5 months before release date and - while I don't know when exactly the first megas were presented for XY - I'm sure they didn't have as much time to feed us with megas back then. So maybe that's why we know so many megas already.
As much as I love new megas and the diversity they bring, I'd personally prefer if GF didn't overdo it. But that's not how things are shaping up to be. If I recall correctly, GF explicitly said "hey guys, 5th Gen pokémon can mega evolve too" as they introduced mega Audino, which hints to a couple more Gen 5 mega evolutions making their debuts in ORAS. And there are probably a couple more 3rd Gen megas in store (hopefully
Arena Trap Mega Flygon and
Water-Fairy Mega Milotic!). And possible random Gen 1/2/4 megas. So I wouldn't be surprised if we ended up wtih 25~30 new mega evolutions, even though I think 15 would be more than enough.