If the top 4 or 5 leagues agreed to a form of FFP or Salary cap together then it could work. The money on offer in the Premier League and top ends of La Liga, Serie A, Bundesliga and by PSG is liekly far and above what any other clubs worldwide could offer, with perhaps the exception of a handful of middle eastern or Chinese teams and those leagues or the MLS don't have the prestige that would draw the top players in their peak. If UEFA and the ECA agreed on a a real version of FFP and got the likes of PSG and Man City to actually agree to it and abide by it then it would work.
On the other hand, salary caps tend to benefit owners at the expense of players. Ticket prices etc don't go down.
Bookmarks