The leagues have the power on this, it's just whether they have the bottle to use it. There will be a short term, severe financial hit, sure, but ultimately other clubs will fill the void within 4-5 years, because new teams will get the share of the ECL pie.
All it would mean is a much needed financial reset which will slow down football eating itself, which is what was gradually happening and the Super League is the ultimate symptom of it.
If the Premier League and all the others understand that not taking drastic measures is substantially worse than allowing this to go ahead in any way shape or form, then what they need to do is very obvious - they need to revoke the eligibility of all these clubs to play in all existing competitions immediately, ban their players from representing their nations, and give a deadline to drop the entire thing before they are permanently excluded from football. It's the only way.