Well, no one bats an eye if a manager chooses to become an IC (of course they have to know their shit, many of whom don't, and always bluff "I wish I was an IC"

Well, no one bats an eye if a manager chooses to become an IC (of course they have to know their shit, many of whom don't, and always bluff "I wish I was an IC"), but from IC to manager is not an automatic option. Being a manager is "not a promotion" promotion.