rdj as doom

can someone genuinely explain why marvel chose robert downey jr to play dr doom in doomsday and secret wars?? apart from the nostalgia bait, like is he meant to be a tony stark varient or something? I DONT GET IT. there has never been a comic run or anything to do with victor being a varient of tony (to my knowledge correct me if im wrong) so i don't know why they would recast downey. im sure rdj is gonna be great as doom nonetheless but it all feels like a cashgrab, i wanted doom to be organically introduced but without any buildup it all just feels strange. i do understand how they had to scrap all the kang stuff but still im not sure how good doomsday will actually be.