They take a fair chunk ( ~ 25%) of the broadcasting rights, but in return they also provide a moderating influence in favour of smaller clubs and federations. Under the current format, each club that reach the groupstage is guaranteed 15m Euro, and gate receipt for the 3 home matches+ more money for winning/drawing matches. That's a significant chunk of the annual budget for clubs not in the top 4 FAs (or even some already in them). And then with their ~ 800m cut, UEFA execs will pocket the lion share of that, but some will also go into developmental projects that benefit football on the continent.
Now with this Super League, assuming that they get the same money for broadcasting rights (which I suspect wouldnt be the case, as they will demand more money since in theory there are no 'boring' matches), and with only 20 teams participating, you are talking about ~ 200m cut per club if equitably divided, annually. For context, that's double the money Chelsea got as Champions last year, and that amount of money will essentially ensure that the 20 clubs who participate in the inaugural season will keep their place for the foreseeable future, with that competitive advantage over their domestic opponents, bar disastrous mismanagement or state funded clubs upsetting the balance. It would create a new football oligarchy that is ten times worse than the current Champions League.