I must be in the minority but I don't get the disappointment of potentially every Saints player in the squad not being a Saints player by the time the World Cup starts.
It was their time with Saints that got them there. This is surely their pinnacle. They didn't do this with any other team and haven't played for another Premier League team yet, so it is testament to the club.
They could go on to be major flops for their next clubs and may never get the chance to play for their country/in a major international tournament ever again. Unlikely, but could happen.
With their new clubs, their records will still read pld 0 scored 0 until August, so it is still a moment to savour.