If only the new era mattered, bb wouldn't be a main antagonist. He's from a different era, trying to be relevant now. Buggy wouldn't be a yonko, Mihawk would stay semi retired, etc.
I saw Kidd a means to a end, and his end was when luffy became a yonko. Where as a real yonko defeated the would be yonko if buggy didn't exist. Kidd was never meant to be a yonko to oda.