I think he got about 75% of Paxton's value, the Mets deal was pretty fair, and he is heading towards losing some value in this Segura trade if the structure is as reported.
The problem with the Mets deal wasn't the value, it was how he decided to get that value. He decided to dump Cano's contract and shift money onto '19 and '20 instead of maximizing the prospect return he got back. And you're right, he did do that with an eye towards competing in the next couple of years. I think that was a mistake. That Seattle team should have approached this as a complete teardown and done what it could to maximize the prospect capital they got back. But I think their impatience is going to keep them from being truly competitive for a long time.
Plus, I think that they could have moved Cano without having to attach Diaz to him. I think they could have found a team willing to trade two bad contracts in '19 and '20 and have Seattle kick in ~30-40 million instead of the 20 they sent to the Mets. Maybe they would have also had to attach a B or C prospect to that deal as well. Or maybe they could have attached Segura instead. But instead of that they attached Diaz who I thought was their most valuable asset (maybe Haniger).
If they traded Diaz by himself, I think they could have fetched an absolute haul from some team. Especially if the recent reliever market is any indication.