They're betting that, since Harden will be 33 next year, and since their biggest problem is on the defensive end, having multiple years of Simmons under contract was going to be more of a net positive than having to negotiate with Harden (a) if he would sign his player option extension for that final year and (b) if he would be worth what they'd have to pay the year after.

The Nets have Simmons for three more seasons. They'll likely have Durant for the rest of his career with his new deal. They'll either have Kyrie or a lot of money to play with in the offseason. Nets will be fine imo.