Why would a Christian not believe that Prophet Mohammed is not the last and final messenger?
Arguments have always been a means to take people captive. That is all they are good for. We produce a reason not to believe you, then you argue against it. That is how captivity works. Every time the captive seeks to escape the captor punishes until they stop trying.
Is Christianity any different? Currently no. Its mostly the same, and so that is why there is relatively no progress for Islam. You argue. Christians argue. You preach. Christians preach. What is the difference? Islam has progressed mainly through childbirth, people gradually moving in and populating areas. That's how it has grown, and argument is how it retains its people.
Also some of the arguments are really, really terrible. They are designed to keep people in, not to convince people on the outside. Look at the arguments, for example, that the Koran was scientifically advanced. That is a particularly disingenuous and stinky argument. It reeks and is loathsome. The Christians arguments are equally loathsome, so you are equal.