2. Middleton is simply too important for the Bucks to let him walk
Khris Middleton has emerged as a formidable Robin to Giannis Antetokounmpo's Batman over the years, and the playing style of the two has made it quite natural for the two to play off one another. In fact, many believe the Bucks would've reached the NBA Finals in 2021-22 if Middleton remained healthy during the postseason.
If Middleton were to leave the Bucks, their days of being a championship contender would be over, as they need his spacing and would be unable to replace him. This could eventually prompt Antetokounmpo to ask for a trade, which should be the ultimate fear for the Bucks, as a small-market team would never be able to bounce back from losing one of the game's greatest players ever.
And we know superstars in today's game don't necessarily give teams the benefit of the doubt as it pertains to surrounding them with the necessary championship ingredients, as they know their peak isn't going to last forever, thus they need to maximize that window. And Antetokounmpo recently made the Bucks' brass sweat it out prior to signing his extension in 2020, as he waited to extend until after the franchise acquired Jrue Holiday.
Allowing Middleton to slip away would be far too fatal for the Bucks, regardless of the luxury tax implications they'd face if they extended him.