Those teams will still compete domestically, their players will still play for their national teams...nobody will notice the difference.

You're leaving out that the new larger Super League would become the priority tournament year round. This would make the Premier League (the most popular, lucrative domestic soccer league in the world) into an afterthought, similar to the currently stagnant Carabao Cup. Teams in the Super League could afford massive squads, with the best players being reserved for the Super League, and the formerly number one Premier League being used to play fringe players.

Because of the fact that - unlike in the entire European football pyramid - the fifteen founder teams would not require any qualification via domestic league status, there will be no incentive whatsoever to risk major players in the Premier League, or to make serious efforts to win it.

It's a closed shop. Forever. No Leicester City miracles ever again.