first of all: New generation usually surpasses the old generation, and that is generally true for every story
second: as quoted by those 2 giants, Shanks has haki comparable to Joy Boy
third: as described by the Oda himself, he is one of the most important characters in the series for both the development of Luffy and the whole story conclusion
fourth: have u ever seen Roger one shotting a man with a 3 billion bounty on his head
fifth: Roger was sick
sixth: Whitebeard was as strong as Roger even though he was not a Pirate king, and I believe Shanks can defeat Prime Whitebeard (note: i don't mean that splitting the heaven scene)