"There are two reasons for this. First, League of Legends sometimes can become 'snowballey'. Some team compositions especially, if they start doing well, will keep doing well. **We've seen internal games where you get a 25-5 game and then with the same teams get the opposite result next time around. **" Game designers obviously do not entirely agree with your statement.
Same team, not same team compositions. And main idea was about "snowballing". I can't find this season's interview with one of game designers, who said that new meta is based on early domination, and Riot must fix it. After that we get Eve, Elise, Lee Sin, Kha, Ziggs and LB nerfs.
Um, honestly, your argument is really weak. First you claim MOST pro-players recommend playing snowball champions, to get higher in ranked, HOWEVER you have absolutely no proof that most pro-players agree with you. What is a snowball champion anyways? Some pro-players may have different opinions on what it might be, meaning that what you think is a snowball champion might not be for a pro.
http://www.lolpro.com/guides/game-play/212-gameplay-guide-how-to-escape-elo-hell/mechanics-skill-sets
http://www.reddit.com/r/leagueoflegends/comments/1llz77/meteos_ama/cc0q746
http://www.reddit.com/r/leagueoflegends/comments/1wq8st/gambit_alex_ich_ama/cf4fhu1
Snowballing, carrying 1v9 - imho very clear explanation.
Finally you make a wild claim that adc/top/mid will grow much faster than support/tank, without any proof.
http://na.op.gg/ranking/ladder/ - how many supports can you find here?
If you look closely - best winratios and least amount of games (to get back into Challenger) have players who played assassins/adcs.
You raise higher in the ranks by winning against higher ranked players. To raise faster, you need to win more. Sure some roles, and champions will help you win more. However it is impossible to make them all equal, because different players have different win rates with certain roles and champions. 1 player may have a 90% win rate with support and a 10% win rate with adc, obviously buff adc and nerf support based on that will break the game for someone else.
I wrote clear explanation for that already:
*my young brother is Diamond 1. He made another account and picked Tristana in his first ranked game. He did penta and had score like 35-2-15. Would he win this game if he played support? I'm not sure.
*
So different roles do not need to have to be the same. That'll just be boring.
I was talking about time that different roles requires to become Diamond (at least) in full of chaos solo-ranked.