Math time: Mamdani took more than 50% of the vote, so there's no way that removing one of the other two candidates would have hurt him. In all probability, if either Cuomo or Sliwa had gotten out of the race, both some of their dedicated voters would have stayed home, effectively increasing Mamdani's share by shrinking the whole number of voters, or some of their voters would have chosen him instead of the other candidate, increasing his percentage of the vote again.