To be honest it’s Real and Barca that need this the most because they’re heavily in debt and the pandemic has has hit them harder in their ability to spend insane amounts of money on the top flight talent they need. They’re not owned by Americans.
My understanding there's 3 groups within these so called Founding Members:
1. The American owners - Fenway Sports Group (Liverpool), Stan Kroenke (Arsenal), the Glazers (Man United). These are venture capital/private equity/hedge fund/ guys, with the advent of the Premier League and all the money it generated, they bought the teams as business investments and want to generate profits, which means increasing revenues and reducing costs. I guess you could add Tottenham, which is owned by an investment group headed by a guy that lives in the Bahamas. They hate the fact that they have to qualify for the Champions League every year, because that determines whether they make a profit or not. They also hate the fact that there's an arms race in terms of spending to get top shelf players and no cost controls. They've grown up on earlier investments in sports teams in American leagues, like the Bucs, the Red Sox, the Rams, the Nuggets, in leagues which have salary caps or cost controls in some shape or form, guaranteed participation in the highest forms of the game and thus have more stable cash flows. The Super League would have been more like an American league with predictable revenues and ROI. Even if you come dead last, you're still in an exclusive league year on year with all the solidarity payments that JP Morgan committed.
2. The oligarchs/oil money owners Abramovich (Chelsea), Sheikh Mansour and Abu Dhabi United Group (Man City)These owners could care less if they make a profit. They're in it for publicity and because it's either a hobby (Abramovich) or it's something that makes the owner/country of the owner look good (Sheikh Mansour and Abu Dhabi). They'll spend whatever money is needed to get the best players and if it means they lose money so be it, there's more petrodollars from either Russia or the Gulf to cover it anyway. It falls on UEFA or the domestic league administrators to try and rein them in with things like Financial Fair Play (an oxymoron if there ever was one). They really didn't need to be in a Super League but they didn't want to miss out. It's not surprising that they were two of the first to leave.
3. Owners of clubs not in England - including member owned clubs like Real Madrid, Atletico and Barcelona, as well as ones owned by other billionaires or investment groups, who like group 1 want to make money (the Milan clubs owned by Chinese and American private equity) as well as ones are too emotionally or culturally attached to their team to consider divesting it e.g. the Agnellis and Juve. They don't have that Premier League money to help with their topline but they need to keep up so they're heavily in debt and Covid has made it worse. They've really been pushing for a Super League because they're at a financial disadvantage compared to the English clubs and they can't keep throwing ridiculous amounts of money to buy players because they'll go bankrupt. It's also not surprising that they are still the ones that are in there, other than a couple of them who left. Real, Barca, Juve, they need this Super League the most. Financially they are on the brink.
It's interesting how a bunch of owners with such disparate goals ended up coming together, but the overriding goal was to get a bigger slice of the pie for themselves. Some needed it more than others but it was obvious that the entire setup was fragile.