You're missing the point.
Maybe he doesn't deserve it, maybe 50-50 is fair, maybe it's the most money he'll get from a fight etc etc etc. It doesn't matter because he knows Brook needs it more and he knows he brings more money to the table.
The peanuts Khan will make fighting in the US is more than what Brook will make fighting anybody other than Khan but more importantly, Brook isn't financially set for life like Khan is. For over a year all you've heard from Eddie Hearn is how he needs to get Brook the big pay days he deserves - the problem is Brook isn't the big draw, they need somebody to bring the money to the table and there's very few options.
If and when this fight is made I'd bet my life that Khan walks away with a bigger cut than Brook. His team have opened up by asking for 70% knowing that Brook/Hearn will try to knock them down. It's the same with Brook/Hearn opening with 50% - they know Khan will want to knock them down. That's not me saying he deserves it based on his boxing but it's how it will play out, I'm certain of that.
It seems it's you missing the point, Khan needs to do something. Turning down Brook and fighting another nobody a terrible move. If Khan gets dropped by nobody again it's career over and if he wins who cares or even bothers to watch? After that nothing fight, it's the winner of Thurman v Garcia and a loss would mean Khan probably drops out of the rankings.