It's not cutting your nose off to spite your face if there is legitimate competition for your asset. Even Siakam was already 30 when the Pacers acquired him, knowing they'd have to spend a bunch of money to re-sign a third-tier star without assurance that he was their "one-player-away" from reaching the Finals. Fox is just entering his prime, having turned 27 in December. There's a reason half the league has already called the Kings to inquire about his availability. 25 ppg scorers with a talent for bending defenses at the rim and a finishing ability like few others in the league is not something you sell for less than its value. You certainly don't take a crummy deal today that you can also get in the off-season when other interested franchises will come to the table knowing they can beat that crummy deal. Rental or not for some future trade partner, Fox is the kind of talent at this stage of his career that quite a few teams would surely think of as their "one-player-away". Kings fans need to shake off the hangover of Vlade "better deal yesterday" Divac. Monte absolutely should not be moving De'Aaron Fox for pennies on the dollar.
What's crummy though? Like I said, taking a decent deal should be the goal if there aren't any great deals. If a team is willing to gamble, great, but with half the league calling there's already a robust market it would seem.