It will ruin domestic football!
It will end meritocracy. At a time when we've almost got rid of most of the old Sky4 from the Top4 (this season could easily see City Leicester and West Ham finish in the Top4) after a decade where Chelsea have finished outside the Top 4 four times, Arsenal 5, United 6 and Liverpool 7... that shake up of the Cartel that we started will be over forever. and we've just become part of the Cartel.
Leicester, West Ham, Villa, Everton, Sheffield Wednesday, Sunderland etc... what's the point for any of them anymore? They can perform brilliantly in the Prem and not be invited to this Super League if the Cartel don't fancy them, and there's no point in ever being invested in if you're Wednesday or Sunderland because the dream to do what City have done, or even to a lesser extent what Leicester have done, is gone forever.
Premier League games will be insignificant to Super League teams, making a mockery of the Prem. It won't matter if any of the Super League clubs win the Prem or finish mid table every year, they are in the Super League no matter what. The clubs can even use the Prem as a reserve competition, resting their best players in every Prem game ahead of the money spinning Super League games, because it doesn't matter where any of them finish in the Prem. The Prem will lose all its prestige and will be finished as the brilliant world leading competition it is... forever!
And let's not kid ourselves that the knock-on effect of that won't be that the Super League Cartel won't eventually make the Super League games the weekend games. This is where this will head in the end. News coming out already that the owners don't even care if they are banned from domestic competitions because the Super League will be so lucrative.