Kaku might be popular, but pulling a Robin would just kill of some interest in One Piece…...
No, those are different cases.
First, Robin was an assassin in an illegal criminal organisation, while Kaku was an assassin in a secret service LEGALISED by the world government.
Though we haven't know whether they ever really killed anyone yet, being secretive is dark enough as they have to live under 'shadows'. But being illegal would be a different matter than being legal.
Second, Robin was partial to Mugiwara since the beginning, might be because of the "D". She let Mr.8 went unscratched, she showed an alternate sailing route, and then she helped Luffy through painful moments. Kaku seemed to make the situation worse by dumping Merry (though that'd be the best thing for them as Franky also tried to rip her off), although later with his sportive attitude he gave the right key to unlock Robin when the CP9 could've cheated by hiding it somewhere else.
Third, Robin was never actually lose to anyone on the mugiwara in a one-to-one combat match, though she always refuses when being asked by Nami to force Luffy&co on some occasion. While Kaku, as an already defeated one (we could blame the new fruit or the determination to win), would create a different atmosphere than the Zoro-Sanji rivalry.
And the defeat would become the very turning point for him to stop being WG's pet and shout out loud to the world: I am a real boy!
Dissapointed being fired by Paulie, Kaku would start looking for a new interesting job: not a jokingly zoo, but a more challenging one such as a pirate.
