I see two flaws in your wall of text.
1) The Exp. Share is actually in the game, be it extremely late.
2) There's a point as to why the Exp. Share is of no use early on, you're limited to level 15 before facing Julia, 25 before facing Florinia, etc.
The reason Ame made it this way is to actually make you work with what you can get at that point in the game and not make the game ridiculously easy, like X/Y's Exp. Share benefits. What fun is it to get your teams leveled up insanely fast without barely doing anything for it. This is also the reason why certain Gym Leaders do not give you badges, I believe. If you were to get every badge up to Kiki, you would have..5 badges, which would make your Pokémon obey up until 65. I honestly don't see anyone having level 65 Pokémon after Kiki. In my original playthrough, I had trouble having my team under the level cap for Noel, which was 55 or 60 I believe, without grinding all that much, yet having hard-to-raise Pokémon such as Gyarados and Excadrill. If we were to really get the Exp. Share early in the game, be it after Corey, people would have to scavange for money to be able to afford the Common Candies to stay under a Gym level cap before getting the next badge. I can know, because without training much, I have an Azumarill about to hit 45 before facing Kiki. Granted Azumarill levels ridiculously fast due to its exp curve, which brings me to my next point of interest; Some Pokémon level up way faster then other Pokémon do, again pointing to the fact the player would require heaps of money to afford Common Candies.
I believe the current Exp. Share is better because it splits Exp. Points between two Pokémon only, meaning you can slowly level up a strong Pokémon while leveling something freshly obtained/bred.
But that's just one person's opinion.