This is exactly why the on/off stat doesn't measure impact. You need measure what would happen if the player was replaced by the average player in that position. Instead it measures what happens when you are not on the court, when multiple players are subbed and typically those 5s play just a few minutes together adding more randomness to the results. I always give the same example. If the team had a second Sengun, to replace Sengun he would have a significantly different on/off number because he would be playing with different 5s, although they are exactly the same player.
That cap space can also be used other ways, not just FA. Extending Sengun would be basketball malpractice and the absolute height of idiocy.
Sengun either gets the max now or he gets the max a year from now. With his play, it makes zero difference, he's getting the max barring career altering injury.
About the contract situation he explaines how the rules work and just said " We are waiting, my agents are negotiating with them. Yes I want to get the contract now, because it clears your mind". I don't really think there is an offer on the table but it is sure he would like to get one as early as possible.
It makes absolutely no sense to max out a guy early and completely give up the opportunity to use cap space. Even if we aren't able to add a superstar via free agency, there is still a ton you can do to add significant talent. It's been said a billion times, but the 3 ball would instantly make him a true superstar. He can't fix his verticality issues, but I think that can be managed. Roster building is so tricky with one non shooting threat already baked in. He can still be a star without it, but not a guy you build a title contender around.
I would have zero issues with them maxing him out now. He is clearly the best player from his draft class and lesser players have already been maxed. I understand the cap ramifications and why they likely will not do it until after the season but there is zero doubt he has earned a max contract. If he was picked #2 rather than #16 he probably would be getting the max extension sooner rather than later.
He isn't good enough currently to demand a max. He might get one cause everyone gets one. This is how teams get stuck on mediocrity treadmill. Giving a good player max money.