They can't delete champs, people find them fun to play, and people main these champions, spend money on them, devote there lives to it (RedMercy, Trick2G, etc.). These guys (Except Zed in my option) are the flavor of the month champions. I remember when both Udyr and Yi were not picked or worried about. These guys have counter-play in ether itemization or playstyles you need to adapt to play against them. Now if your playing an immobile squishy take my main for example Varus you are in a non favorable match up and thus you are playing to there strengths, Zed isn't strong right now, i don't care what you say he is weak, he may be save but he is pretty weak in this meta of bruiser agro-magnets he cant deal with them efficiently, but he will still see play because he has a fun flashy play making kit that makes you feel powerful and gives you outplay potential. He has insane amounts of counterplay in terms of champions, items, mechanics, matchups, meta he is a healthy assassin(Riot has stated this many times). The other two are counterable as well. Yi CC him. he can't do much. Udyr kit him to death. With all the flavor of the month champions you need to learn and adapt to playing against them instead of crying to for forums to remove them. Sigh alright i cant say i've never gotten angry about a certain champion in the game but this isn't a good approach to it. If you truly want to grow as a player and become a little more mature don't say "GG guys FF at 20 they have "X" champion" this isn't the mindset for winning or climbing elo, this will drop your elo faster then you can say "mid or feed". If your first thought when you see "X" champion is to FF then no wonder your losing, you need to stop that and start thinking "how will i deal with them and win". Yi has always been seen as broken, newsflash he's not. he is an assassin who is semi-abusing a favorable item build to yield the most profit, this isn't Yi's fault, you probably weren't around for AP Yi, that was broken it wasn't meant to be so they removed it. the Yi we have now is but a shadow of that. Udyr isn't my strong suit but I've seen many people deal with him and counter and beat him. He is in the same boat as Yi, not broken but abusing a certain item build. If you remember the Udyr were you could buy 2+ Jungle items on him that was scary compared to this.
My Best advise is start playing these champions and learning them and there weakness so you can exploit it in future games, good luck and please try to keep the flaming to a minimum ^_^