if we are talking strictly about national team play, real world too is unrealistic.
until 1984 you had non proffesionals in the olympics. now 1 team has total salaries of all other countries combined. as long as players cannot control where they want to go, just like free agencyin nba or european leagues, you cannot complain abiut low ranked club's ability to retain superstars.
and at least for u21, i find it realistc. many clubs , even in lower league, hold on to a player until he hits 21 to sell him off as a young star, rather than sell him off as a junior/cadet. why would Luka Doncic ever play in his non naitive country for a fraction of what he would make in the states.
otherwise imagine how countries would feel if you could nationalize players, granted they would never be top 30 nt player but they would be a superduperstar on some underdeveloped countries
i think it works as is. this games does not need to mimic real life. i think that gives it alot of charm.