- There is no Lv100 option in XY battles: Heart Gold and Soul Silver were the last Pokémon games to support an auto-Lv100 option. Since then, cartridge players have had to rely on Action Replays to level their Pokémon to Lv100 for competitive play. We kept the auto-level 100 ruleset last generation, but is it really worth widening the gap between simulator and Wi-Fi players?
If you wanted to use this argument, you should have used it last generation. The precedent has already been set. While players cannot rely on AR to get their level 100 pokemon yet, in two months, they will be able to transfer them over from previous games. This makes 6th gen pokemon the only ones you have to spend a few hours grinding for. Maintaining Lv100 metagames did not hinder Wi-Fi players last generation, and it won't this generation either.
- All cartridge and officially supported metagames are Lv50: As previously mentioned, all XY Wi-Fi battles will be at Lv50. You will never find two players who legitimately leveled all six of their Pokémon to Lv100 in order to fight each other. GBU and VGC already use the LV50 format; many players are already familiar with it.
Before anything, i want to highlight 'many players are already familiar with [Lv50 formats (including speed tiers, EV spreads, scarf magnezone outspeeding your starmie)]' No. No they are not. How many people play VGC? GBU? Compared to the amount of people that play NU, RU, UU, OU, Ubers, and any other oddball Lv100 metagame? Making lv50 the default, as you've addressed, would force every one of those players to relearn every stat of every Pokemon. Keeping lv100 the default only requires players to learn the new pokemons stats. That is a lot more convenient than anything you get from making the tiers lv50.
But, moving on. How often did we get to see matches where players leveled their whole team legitimately last generation? I mean honestly, any generation after GameShark? Competitive Wi-Fi players always use hacks (ar/gs) to obtain their pokemon, and it won't be any different this generation, once the codes come. Again, in two months, we will be able to transfer our Lv100 pokemon from games where you can hack rare candies, if not hacked pokemon themselves.