It's supposed to be the best of the best in the country, who gives a shit if they're glory hunting, that completely misses the point of this tournament. If people come back further into the tournament and can improve their nation's roster it will only improve the competitive level of the tournament as a whole.
If someone coming in late 'completely affects a match' then that is fine as the entire point of this tournament is to field the best team the nation can field, not the best team that they're allowed to field because of some arbitrary roster rule. Say Germany starts off without Wolpi cus he's inactive and he comes back to the game during quarters, why prevent them from having a stronger team and a better chance to win with him in it?
The seeding argument also makes 0 sense as the current rule doesn't protect the seeding in the first place. Seeding is only relevant during the groups and noone is going to make more than 3 subs that early on into the tournament. So seeding can be 'affected' as much under the current rule as without it. And let's be honest, noone is going to impact a team so much that the seeding suddenly becomes completely imbalanced.