And also gaining more rewards for themselves I think those low-level players would rather have a chance at more diamonds than not. There’s only so many people in a Challenger league - having more people overall wouldn’t make it easier for the top players, it would just increase the number of leagues in each bracket.
With regards the difficulty, I agree that it’s supposed to be difficult, but there’s a balancing point - often after week 2 I don’t really bother trying in Coliseum, because the number of teams far superior to mine makes it not worth the time & effort (and diamonds!) to try to place highly.
The problem with things like “make it so you only need 2/3 to win” is that also causes defence to be a very different proposition, likely actually to the detriment of weaker players. Many can’t create 3 strong teams, so put 1 or 2 stacked teams and the 3rd is meh - which is fine under the current system, but wouldn’t work nearly as well if only 2 wins were needed. They’d have to spread out and probably lose more defences.
I do think Coliseum needs some consideration as from Challenger 3 upwards it’s basically P2P, and my experience is that below the top 3-4 people per league very few actually bother to use their keys.