The year-to-year is pretty noisy so the four-year MA is more discernible, but at a glance they paint the same picture. I think it's a structural change on two fronts. Aggressiveness/style of offenses, and quality of QBs. The style of play is fairly self-evident, I think. IMO the game has more QBs now who are more capable on average than the QBs on average from when your chart starts. Start with the QB running. This was more of a novelty 20 years ago even though there have always been guys who could run (this doesn't apply to option offenses, obviously). Today it's not only common, it's probably prevalent for a QB to be a good runner with designed runs factored in, which makes it a lot harder for defenses to stop the chains from moving. It also used to be the case that in many teams' offensive schemes, the QB was only responsible for reading half the field on a given play. Whether or not they were capable of more is moot, what matters is what they were doing. I can't be sure about this, but today's game regularly has QBs who appear to be reading the whole field routinely. Or they're supposed to be. The ones who can do it effectively become your best guys. If I'm right about that, that's a stress on defenses that didn't used to be there.
But your question was does defense still win championships. I'd posit that yes it does, as it arguably matters now more than ever because teams can score so much. You need to be a team that can get a couple stops against a great offense. Maybe not shut it down, but not allow the opponent to score every drive.
