So to expand upon my earlier remark..
I've now been playing League for a year and probably the first 8 months I was fearing the saturation of champions into the game but more recently I've seen how well Riot has balanced new champs and how the number of champions really doesn't affect much.
There are definitely some underpowered champions but on the whole most are balanced. What's deemed as OP can change on a whim from what I've seen. Usually OPness is determined from the competitive scene. There are certainly some "very good" champions, but everything has a counter and everything requires execution. While most of solo-Q fears a Blitzcrank support, he isn't OP if you are aware and can dodge his Qs. He is a 1-trick pony and if the player can't land a Q then he's worthless. Blitz CAN decide a match if played well, which is why he is often banned. The luck-factor of whether you'll be against a good or bad blitz is too much for most people and so he's often banned.
Back to the original point though... I don't see a cap limit on champions coming anytime soon. Most new champions simply shuffle the deck of existing tools and rarely bring anything new to the table. What's the difference if there are 100 champions or 1000? As long as everything is horizontally balanced then there isn't much to worry about. Most players focus on only a select few champions anyway. There is no pressure to master more than a few champs. Everything considered: having more champs doesn't affect anything.
It might be overwhelming to know what every champ does but play enough and you can figure it out easily enough. Regarding the pro scene: the majority of champions are used, which is great. Pro-bans often focus on banning out the champs that are the favorites of the opposing team, like banning out Anivia from Froggen as example.
Since players will strive to master only a few champions, this tactic will remain and keeps the game exciting. Having 100 or even 1000 champions won't change how the banning phase works. Froggen will remain as an amazing Anivia player and will likely see Anivia banned out against him more often then not. Future pro players may master other champs and be renown for whatever champ they decide to play.
I played the original GuildWars for 2 years. I compare it to LoL a lot since both LoL and GW revolve around creating a team "build" that is cohesive. GW eventually failed due to too many skills & classes and this is what has spurred me to beworried about LoL undergoing the same fate. But I've changed my mind on the matter.
There is a massive difference between the games: LoL has a pick & ban phase where teams can pick & counterpick each other. This keeps games extremely balanced. GW never had this. Often times it was a game of luck. You had to get lucky that you weren't going against a team that countered you. Example: Your team is a Hex heavy build, enemy team brings many hard counters to hex-builds.
LoL can have as many champions as it wants without affecting too much since the pick & ban phase eliminates any "luck" factor. It is certainly possible a player could counterpick themselves, but at least it isn't a surprise when it happens.
LoL has slowed down their release schedule which is good. I like keeping champs fresh and relevant for the most part. At their current pace they will release about 17-18 champs a year. At this pace LoL could potentially have a total of ~300 champions in 10 years. That's very manageable imo. The game won't implode and the power-creep should be minimal. Anything that falls from power will likely get a rework and become relevant again (Jax, Eve, Twitch, Kat, etc).
Like I said, first 8 months I was extremely worried, but now I embrace every new champ and am just searching for the few that appeal to me most and that I deem fun to play.
