Assuming you're genuinely asking for the perspective behind this argument. I think these folks* mostly believe that cleansing is a "least bad" alternative because Palestine is a failed state with broad anti-Semitism and a Jewish neighboring state. No group in Palestine (even Hamas) is capable of enforcing a peace treaty, and Israel is not willing to abide by a peace treaty that Palestine groups do not abide by. Thus, the status quo of attacks from Palestine to Israel by various armed gangs will continue indefinitely, and the counter-cycle of Israeli retaliation, a counter-cycle which inevitable undermines Palestinian groups that deal with Israel. The most likely possible disruptions of this cycle are tragic - either the rise of a right wing group in Israel that cleanses Palestine or the regional hegemony of a current group in Iran that cleanses Israel - and even assuming one of these tragic disruptions doesn't occur, the status quo cycle is already pretty bad.

* I say "these folks" because I don't know that I agree.

They can, if they simply stop launching rockets aimed at civilians. Literally, that’s all that needs to happen. Rockets stop, bam two state solution next day.