Well Cuomo is running to try to re-enter politics that was obvious when he ran in the democrat primary, and they rejected him pretty soundly and nominated Mamdani instead. Cuomo then dropped out of the democrat party, an old time esttablisment democrat from an elite democrat family, in order to run again against Mamdani. It is personal with him and ties into his ego and his family legacy. For Sliwa as he even points out in the debate he does not like Cuomo either, he won't drop out just because he personally does not like Cuomo and thinks of him as one of the worst Governors in NY history which is not actually far from the truth. So it is that Mamdani will win with them divided on eachother, but even if we entertain the hypothetical of one dropping out and outright endorsing the other or making some sort of 11th hour alliance, Mamdani still will win, especially after this debate where one or the other had to really lay a finisher move on him, but both failed to do so. Don't get me wrong either Mamdani wasn't so perfect in this debate and they did land some blows, Mamdani did get staggered and stumbled a bit more in this debate, but it wouldn't be enough to move the needle enough when he already has a pretty commanding lead in the polls.
All this said though there's still a technical chance still that Cuomo could win, and an even slimmer chance that Sliwa could win, as unlikely as it could seem it is possible, but not very probable in either case. They have by election day in a little under two weeks to figure somethng out, and political comebacks and underdog upsets have happened before of course. Don't get all your hopes up either though, the writing seems to be very much on the wall now after seeing that debate, considering the current opinion polling, and with the limited time available until the election.