The Looker chapters are excellent, and a great way for the game to make a quest differently from "defeat eight gyms and the bad team". It's possibly the greatest story telling in a main series Pokémon game to date.
That being said, the post-game content of XY is rather scarce. The Battle Maison gets slightly boring after a while, the Elite Four never get any harder, and finding the Mega Stones doesn't take too long. Friend Safari in Kiloude City is a great touch, but ultimately doesn't extend gameplay all that much. I suspect this is deliberate.
The business model of GameFreak does not rely on game longevity. Trying to extend the main game is hopeless, since after a while you'll have strong enough Pokémon to plow through anything the game can reasonably throw your way. Upping the difficulty by adding really-hard-to-beat opponents would eventually exclude less experienced players. Instead, GameFreak rely heavily on making entirely new games from scratch, and give players a new adventure to play. I think that some time during the next 4-6 months, we'll see a new Pokémon game being announced or at the very least teased. By then, most people who played XY will have "exhausted" the game somewhat, they have seen everything the game has to offer, and given most if not all of its aspects a try. The heavy-grindy stuff or collecting every Pokémon aside, the average player will have been through most of the game content and hungry for more.
That's when XY2/Z enters the picture. It offers all the excitement you had with X and/or Y, and then some. Extra features. Better battle facilities. Snazzy new decorations and additions to existing towns. A wider Pokémon selection, including a couple of legendaries from earlier generations. New locations. Better gyms. Perhaps an intriguing change to an existing mechanic. And, lo and behold, more post game content, enough to keep you satisfied all the way until the next generation rolls around. All in all a better game than X and Y were, and well worth dropping another NOK 399,- at. We'll be happy, GameFreak will profit, and new fans will be drawn to the series.
There's a reason why the features mentioned above weren't implemented in XY from the start. By holding back the bells and whistles (and keeping the limit of one save file per cartridge), GameFreak will have plenty of stuff to add to the third game/sequel pair to make it look (and be) better than XY. We're supposed to go tired after a while, so we eventually want to buy more games. And actually, I'm not complaining. I like starting a new Pokémon adventure, but I do not like to erase the progress I have already made. I can happily afford to buy a new game every 15 months, to get a new and better adventure than the last one. I look forward to X2Y2/Z, and in the mean time, I'll try to get as much fun as possible out of X and Y.