They largly existed in an era where voters knew little or nothing of down ticket races or down ticket candidates. That isn't the case today with Senators spending at least 15 mil to get elected and House memebers at least 3. They get their own message out.
If coattails existed Bush wouldn't have won NC by double digits in the same year that Democrats in NC extended control of the State House and Senate.
If coattails existed Republicans wouldn't control 8 out of 11 House seats in MI while Democrats win the top of the ticket.