8. A Major Hero Has To Go
In the past, Marvel haven't exactly been committed to killing off their heroes, and when they have, they haven't stayed dead. Agent Coulson returned in Agents of S.H.I.E.L.D., Nick Fury was dead-but-not in The Winter Soldier, and somehow Civil War passed without any major casualties. The biggest deaths in the MCU so far have been Quicksilver, in the same movie he was introduced in, and Peggy Carter, who died of old age.
Infinity War needs to show it has stakes - even though there's still a chance whoever dies WILL return for Avengers 4 - and that means one of the absolute biggest heroes, i.e. an original Avenger, has to go.
For the necessary weight, that ultimately means one from Captain America, Iron Man, and Thor will bite the dust, so the odds are already against Cap.