No silly question at all. What the Corleones did was shock and awe. The novel states that shortly after the massacre the most important caporegimes in the Tattaglia and Barzini families sided with the Corleones. But it's also stated that hostilities continued for another year. I'm sure that after the massacre Michael had made a lot of enemies. But I think nobody was willing to challenge the most powerful family in the country. The Corleones cut the head of the snake. The rest just went along.

Also, I'm sure a lot of people profited from the massacre. In the mob, everyone is luring for the top spot.