What’s there even to play for in such a league? Money? A trophy?
In the Champions League you can claim supremacy in Europe. In the Premier League you can claim supremacy in England. In a closed league you can claim supremacy over 11 other clubs.
Closed leagues like the NFL and NBA work because they are far and away the best league and teams in the world in their respective sports. If this goes through and the PL, UEFA, and FIFA impose the sanctions they’re threatening, I truly believe there is a chance that some of these clubs will no longer even be the best in their home country. In 5 years if Arsenal finish 10th or worse every season, how many players would sacrifice a chance to play for their international side to get beat up on a weekly basis? Money talks, but that money might dry up quick for clubs that don’t have success in this league.