The way I see it is like this, now hear me out ok?
I do understand that its the only free part (that and playing the actual game) but when you get like 100 IP max per game, that's without a boost, and lets say you play even 10 games a day, assuming you win all 10 and get roughly 100 IP per game.
That adds up to 1000 IP right?
I found thread on the league forums, and it would cost you 412,200 IP to buy all the champions, but this thread was posted back in 2013, so I wouldn't doubt that the number has gone up.
Now, with the 1000 IP a day, if you are keeping it free and not spending RP on boosts or something, it is gonna take 412 days right? That's every single day, nonstop for 412 days. The time it takes to play a game is a minimal of 20 minutes, that's 3 and a half hours per day. Not to mention that's with the minimal amount of time, so what if one game takes longer?
There is no possible way, to sit down for hours a day, and play until you win 10 games and get your 1k IP, and like 50 if you lose.
So then whats gonna happen if you lose a lot, or if you cant play as much because of work or something?
That process is gonna take a long time to finish, so long as you are FTP, FTP being you didn't spend RP on boosts or champions. Even then, how many days does that add on? 10? 100? 500?
It would take well over a year, with minimal time played, and 10 wins straight per day, to even start getting the champs you want to play...
This is why I spend RP on champs, and skins alike :/