Balance in this game

Tl;Dr: Dont read...

More Mobility = Less Damage; this is how things should work... Why Riot? Why? Why there are champions in the game that can deal tons of damage and at the same time have tons of mobility? Why should someone pick X champion, that lacks mobility, and need "efford" to do damage with a high risk and not-so-high reward, when you can pick Y champion, that have lots of mobility, and by using to spells can deals tons of damage, stay in a safety range, and in a little time can back to low risk and high reward? Why Y champion, that people dont need that efford to play, even deals more damage than X champion, that also people have to put much more efford to stay at the same level of the other guy?

I think some changes are needed... probably this post will be forgotten quickly, but i got my mind cleaned after posting this... Why there are champions that do more for less, and other that do less for much more? I understand that are 126 (I think) champions in this game, and it may be hard to balance. But, in my opinion are 3 kinds of champions: The healthy, the underpowerfull(I don't know another word, so I will use that), and the broken ones... And some changes between the UP and the broken must be made... I didn't talked about any champion because I dont want this to be just a common "QQ post", anyway, if you ask for examples I will give you them... Also I think that you have some idea of what champions they could be...

P.S: Sorry for the english