His risk was that b/c of his injury near the end of the year, a long term deal was likely unfeasible. If Stone comes back to form with no ill effects from the ACL and MCL tears suffered, then he will get a pretty good payday next year (whether from Arizona or some other team).
And if this year is not as successful b/c of injury, this could be a salary that he tops out at on the open market next year, which still makes Arizona a prime target to retain.
Only wishing that it was a 2 year deal for a little less AAV, but Stone has to maximize his earnings since the injury would likely impact an arbiter's decision.