Remember that Dellacroce and Castellano had a sit down in Brooklyn at Nino Gaggi's bunker like house in Bensonhurst. Castellano conveyed to Dellacroce what Carlo's wishes were and Dellacroce went along. If Dellacroce had been nominated by Gambino and succeeded him, he still would have died of cancer when he did, and Castellano still would have been next in line. It might have changed the whole dynamic that made Paul so unpopular with rank and file soldiers, as they wouldn't have found out about his affair with the maid, and his prosthetic implant until much later than they did originally.