You CAN'T treat all players the same. If your champion has a healthy 52% win rate in bronze, but a sad 45% win rate in plat and above, it means that the champion has easy access to simple power but does not reward you for improving your skill with them. In fact, as everyone ELSE around you improves somehow the champion feels like they are getting weaker and with no skill expression outlets you soon find yourself stuck with a weak champion instead of the roflstomp engine you used to have.
Now, you ARE right. Some champions are OBJECTIVELY balanced. These champions have decent win rates at the plat and up level where true champion potential is used. BUT sometimes those champions have lower win rates in tiers like silver, due to their potential being behind a skill wall. Now there is a dilemma. If you buff the champion so they are effective in bronze, you are supporting the majority of the player base which is good. At the same time, you have also bumped their Plat+ strength, meaning that when played properly that champion's power is overbearing which is bad.
This is where weird buffs like Vladimir's recent Q alteration come from. Instead of just making his Q stronger, they buffed the normal cast and weakened the empowered cast. This doesn't make Vladimir significantly easier stronger, but it does make him more forgiving to play. They also improved his auto attack animation to let him CS better, which more silver and below players have trouble with where as plat + players have that figured out. These buffs keep his high tier win rates relatively in check, while making his low tier win rates go up. It's a very delicate balance.
Also, we soon forget that pro level play is VERY important to the continued existence of League. If people didn't watch basketball on TV there would be much fewer people getting into the game and the player base would slowly shrink to a much smaller number. Professional play shows how exciting the game can be, and creates celebrities to follow.
If you manage to get your game to such state, you deserve to be fired.
Ok, you know that was a joke right? That doesn't happen any more. League is much better managed now. Go look back at release Xin Zhao and Leblanc to ACTUALLY see numbers that despicable.