What's your opinion about Teemo?

Riot will never remove a character from the game. Anyone who picks that option is either trolling or doesn't understand that little fact.

And I think it's fairly obvious that you've never had to play Teemo against hard CC gapclosing/poking lanes such as Xin Zhao, Nidalee, or Jayce. He burns through mana quickly if he's constantly having to use his W or Q and Shrooms, plus his Shrooms are destroyed when someone walks on them and provide much less vision than a sight ward, making them less useful.