If he throws away his belt then yes he ducks Spence...
It's not even like they would want to marinate a Spence vs Brook fight, it's also not even that they couldn't make Khan vs Brook later... They can make it anytime they want, why not when Brook won't have a mandatory challenger who he already agreed to fight?
Grudge match, blah blah...
Grudge matches are good and they can give plus entertainment and tension but I won't pick a inferior fight just cause "it's a grudge match tho...". Nah fukk that... Quality first, everything else after that.
You basically saying you want that fight more cause you are more interested in the soap opera around the fight than the actual quality of the matchups compared to each other.