I agree. I think it's perfectly understandable why Gambino put Castellano in charge instead of Dellacroce. Dellacroce was a gangster's gangster, but at the time few if any came close to Castellano's business acumen. On paper, the direction Castellano wanted to take the Gambinos in was the right direction; with the family in the front gaining increasing influence in the white collar sphere while in the back you'd still have the traditional blue collar rackets going on albeit in a slightly less abrasive upfront way. There was still plenty of blue collar activity going on under Castellano and it wouldn't have gone away either. He still was an incredibly dangerous guy who ordered a ton of hits and while Dellacroce may have been the authority figure of a certain faction within the family, Castellano still had the backing and support of a ton of old timers, earners as well as hitters.

Castellano's primary mistake was being a greedy fuck (even in comparison to other bosses) and acting like a rich douchebag that already thought he was running a Fortune 500 company. His second major mistake was being a snob to and underestimating the bravado of what was still an important and respected faction of the family.

On paper Carlo Gambino had every reason to appoint Castellano as his successor.