I think the way to look at this from Seattle's perspective is that this amounts to a very, very high risk bet that Cano is going to help turn the franchise into a contender in the near term, and that becoming a contender over the next five years is going to have such a positive impact on their long-term revenue that they'll be able to survive the back end of this deal.
Sometimes I think people on this board understate the importance of future discounting on deals like this. Yes, this signing will almost certainly be a problem for the Seattle Mariners 2023 team, and for that matter, it will probably be a problem as soon as 2019 or so. But if they can establish themselves as a premier franchise over the next few years and build a loyal fan base then it will have been worth the money. With revenues looking to go nowhere but up, especially in a city with a booming economy like Seattle, there's lots of growth potential for this franchise.