Replacing the champions league with a super league is not a bad idea per se. Its' the fact you have 15 teams in it, regardless of how poor they are in their domestic league that is the huge problem. Imagine Arsenal, Spurs knocking around mid table in the premier league with nothing to play for as they're in the super league anyway. Will largely destroy the PL as a competitive spectacle.
If they did this with merit based qualification in the big leagues, the super league could be viable. No more filler games in Europe would be much better to watch.
The way this super league proposal has been gone about also looks utterly reprehensible. I expect it will go down in flames honestly with an ugly battle.