Sadly I think this is a fight that Kell can't win. I don't think there's enough left in the tank. Fair play to him though, it's a better swan song than a bust up with Beefy.
Provided Kell doesn't take too much punishment this might actually be good for him - yeah he can't win but if he fights safety first, lose a decision in a semi respectable manner winning a few rounds here and there, pick up a very nice bag of PPV loot and then rightfully claim that he has no business at 147 anyway and yet still put up a good show against a P4P fighter. Then back up to 154 for a couple of warm up fights for decent money and then a final payday against one of the belt holders at 154, cash out and go home or maybe a final domestic showdown against Liam Smith or similar.
Brook is a glutton for punishment, he will take an absolute beating. But at the end of the day he has mouths to feed like most of us.
Makes sense for Arum and Crawford. Kell is still a name and a former Champion who would look good on Crawford's record, especially a spectacular stoppage. For Kell it's a shot to nothing and he would still have the Smith fight as a farewell banker at Light Middle.