It couldn't ever have been a successful widespread policy with different leagues starting on different dates.... You'd just end up with one mass clusterfuck of proxy-deadlines depending on what league a player's parent club is in
Well most leagues end by the end of May and the first ones start mid Aug. How about having a 10 week transfer window starting June 1st and ending on Aug 15th. Plenty of time for clubs to do deals and managers to get the players they want before their respective domestic seasons start