He'll still be 30, turning 31. As long as he's still a top tier player(Which he really should be but I would say there's definitely a non zero chance he falls off pretty hard especially with the wrist injuries he has had plus normal regression curve), some GM would happily give a 30 year old Matthews a 7 year, $17-18M per year deal when the cap is pushing towards $100M.
Keep in mind the Leafs themselves gave Tavares a 7x$11M deal when he was 28.
I think a 3 year deal would have been maximum savagery from AM34 and his camp, but 4 years could very well end with the same result.