Looks like Milan didn't consider your points before going for Modric. A club should go for a younger player who will play for long in the club. When a young player is growing, he improves in skills and quality but for old players, they become weaker and lose their pace as they grow older. Gone are the days when old players were the best due to experience. Currently, the younger players gat it all.
There are times certain teams take risk on certain player, at that point the players form and skill is wht they count on and will take a chance at that player with the hopes that they will get good replacement in the future after they must have achieved that which they want with the player and moreover the Italian league is not as competitive so modric will cope even with the age factor, at least in the next three seasons he will still be very much active except he gets badly injured.
That's the risk when you take on players like that, the problem is precisely that, the probability that there are serious injuries that then lead to the complete stoppage of the player is there and therefore the club must protect itself otherwise it risks having a hole in the squad.