I doubt it those agents aren’t extending early until they know the salary cap number for that season
If this were actually the case no one would sign long term contracts, but the NHLPA and NHL provide guidance both publicly and privately. Predictions aren't going to be wildly different than reality.
At best for him, the Lightning would have a little bit of extra money to play with in negotiations, but in exchange he'd be jeopardizing his career to injury. Since we have him under control for a minimum of two more seasons, I think his agent would be doing his job to secure his future rather than waiting possibly two more years to get it. If I am JBB, I would be leveraging that in exchange for a friendlier deal. JBB has shown that he likes to do this, so I assume it's happening.
I think GM's have a lot more to lose here, because you're basically providing the player assurance even though they haven't fulfilled their contract. Career-ruining injuries happen, but he's still guaranteed to get paid.