The Champions League is a league comprising of several teams. Each team are champions in their own countries. (Actually membership criteria are no longer quite that strict but that is the original meaning.)
A list comprising of several guests is called a guest list. A guest's list would be a list that belonged to, or pertained to a guest; and a guests' list would belong, or pertain, to several guests. A guest list does not belong to or pertain to a guest, it comprises of guests.
Similarly we might have a book list or an address list.
A league may be considered as comprising of teams, rather than belonging to teams. So considered, a league is like a list.
If each participant in the league was a single individual champion we might call it a champion league, a league comprising of champions. However, each participant is a team, each team individually consists of champions, so it is not a champion league, it is a champions league.
Therefore I think there is nothing grammatically wrong with "Champions League"
Champion'sChampions' League is also possible grammatically meaning that the league belongs to, or pertains to, teams of champions. This is similar to a boys' league being for boys.
As both "Champions League" and "Champions' League" are possible grammatically there is no reason to say that that organisation is misnamed.