Isn't that the same deal Kevin Love got? Without the opt-out?
I don't know why Sacramento gave D.Cousins the max. There is no reason to give him the max at this stage. Just wait until next summer. He would be a restricted free agent anyway. What they gave him would be the maximum he could get then, right? What is there to gain by giving him this deal now? Just wait and reevaluate the situation in 9 months time. No need to jump the gun.
Especially not for an erratic (and volatile) performer who has yet to show himself capable of leading his team to improved W-L records. Unnecessary.
I agree with you completely about him not deserving this deal - and that waiting would be prudent.
But regarding the Love comparison, it's not quite valid because a max contract is often not a market price, and for very good players is actually less than what they'd receive in a free market.
As an example, just because Durant got a max deal, it doesn't necessarily mean that Paul George isn't worth one too, even though Durant is a far superior player. It could just mean that Durant is worth a heck of a lot more than the max.