The thing is, how exactly do you propose balancing the game among the champions?
The way things are, some champions will always be relevant, while others may pop in and out of popularity. In an ever-shifting game where numbers are constantly adjusted, mechanics are added and removed, the items may change, and even the very Summoner's Rift may change.
Yes, Lee Sin can do quite a lot for a champion who has a traditional ultimate (unlike say Nidalee, Elise, or Jayce who sacrifice a traditional ultimate for more abilities), and it can frustrate those who play a champion who lacks the ability to easily clear walls like he can, but that is his gimmick, he can be all over the place. The trade-off is that to make the best of his kit, he has to go all-in, even if it could mean his death at the hands of a better team who can cripple him with just a measly silence? What is a Lee Sin going to do if he tries to Q to the enemy team's Sona, then gets silenced and knocked up by their Cho'Gath, and then cursed by Mordekaiser and turned into a ghost, but other than a Gangplank who runs Cleanse, what champion could deal with that situation?
Lee Sin is extremely ability reliant, a silence alone ruins him in a way it would not ruin, say... a Warwick. A Warwick who is disabled by a silence still has auto-attacks, so in the time a silence begins and ends, he can still auto-attack a few times, which is where the source of most of his damage comes from. The trade-off is that unless the enemy is positioned in such a way, he would not be able to just wall-jump to get to an enemy without his ultimate. The reason Lee Sin is favored in LCS is because at that level, the single largest priority is simply to not die, and Lee Sin is very good as avoiding death via escape, which is why Lee Sin sees more LCS play than Warwick would. Save for a team comp built around a Warwick, or a comp that Warwick heavily compliments, a Lee Sin jungling would be preferred over a Warwick jungle because a Warwick isn't as good as getting away as, say, a Lee Sin would be.
Ahri? Good at escaping.
Leblanc? Good at escaping (why do you think Faker was undefeated with her for the longest time).
Lucian? Good at escaping.
Renekton? Good at escaping.
Riven? Good at escaping.
All those champions I listed are not only good at escaping, their escapes are dashes that don't require an enemy to interact with. A Kalista is not going to escape from the enemy team unless she has someone hard-chasing her who she can kite off of, so her escape is not as favorable as say, a Lucian who has an unconditional dash only limited by mana and eventually only by cooldown. Balancing the game at the LCS level is not the same as balancing at the normal ranked or unranked level, because the mentalities are so different that you might as well be playing another game.
An LCS team would rather a strategy with 90% effectiveness even if it never got them a single kill on the enemy team, rather than a 50% effectiveness strategy of simply fighting the enemy team. Their desire to win supersedes any arbitrary score like a KDA, your KDA of 40/0/20 is not the same as last-hitting the enemy nexus. At our level? We aren't in the LCS, we don't get anything beyond an increase in our rank (in ranked play only) or a sense of personal accomplishment, so we seek to get the best KDA because it's the only way to validate ourselves, deep down regardless of what we say, most of us often would rather consistently go 20/5/10 and lose than go 2/4/5 consistently and win, even if this was to continue for the end of time.
That is why it's impossible to compare balance around LCS with balance around normal/ranked.
