They're going to lose a huge amount of fans and therefore revenue if they go forward with the Superleague. No real fans will want to watch a glorified friendlies league.
People will end up watching. It would kill the CL , people will hate it in the beginning until they get used to it.
Pretty much what D2M said.
And, let's be honest, the Super League is more about the international money than it is about domestic money. Biggest backlash will happen domestically. The founding clubs are the biggest brands in club football with the largest international followings. Losing Messi might hurt Barca's brand internationally though. It is still a top 2 brand, though.
In the USA, mainstream sports writers mainly thought it was a good idea. India, USA, China, Middle East, etc. TV contracts. That's the name of the game. It is for the CL and it would be for the Super League. Biggest difference being that the clubs and not UEFA would control the revenue streams (TV contracts, sponsors, etc). That's essentially what they want. Cut the UEFA out and GUARANTEE revenue (no pro-rel for THEM - which is why clubs like Arsenal and Tottenham were interested. Heck, all the English clubs run the risk of missing the CL in any given year. Chelsea has, Liverpool, United has, Tottenham and Arsenal do regularly. City might be the only exception, but f*** 'em
).
Reason why the English clubs quickly backed out is that they are already getting a lot of international TV money and had the most to lose by going to war with their federation. More than La Liga clubs and certainly a LOT more than Italian clubs. Barca, Real and Juve did not back out because they have the more to gain with the Super League. They are huge brands who play in leagues that do not get as much attention as the EPL. Heck, I'd say that those three are bigger brands than any of the English clubs. The gap is closing though.