Yes, if you could do a "big bang" migration at once, egress fees are not a problem. I. Reality, most migrations happen but by bit over a long time - draining a lot of egress fees.
But more fundamentally, egress fees make it harder to compete with AWS - unless you provide everything AWS does, onboarding customers is hard. "Oh we'd like to use your innovative X, but you don't have AWS Z, it becomes expensive with egress fees"