Nations do not wink out of existence. They do not pack and up dissolve themselves, dispersing their populations throughout the other nations of the world. Once a nation is founded — not just as an idea but as a concrete entity taking up physical space and defining and defending its borders — it becomes a community of people that acts out of self-interest and displays a will to self-preservation.
The only way for an actually existing nation to cease to exist is for it to be invaded, conquered, forced to surrender, annexed, or suffer some combination of these or other military actions against it. The only way for an actually existing nation to cease to exist, in other words, is for it to be defeated in war.
To reject Zionism is, in effect, to give a green light to such a war against Israel, by anyone at any time. It is to eliminate the need for a justification for war beyond the mere existence and presence of Israel on the eastern shores of the Mediterranean Sea. A possible attacker — a freelance terrorist, an operative of an organized terror group, a nation, or an alliance of likeminded nations — need not construct a rationale beyond giving effect to the fact of Israel’s illegitimacy: its founding was never justified in the first place, it shouldn’t exist now, and any attack on it that might take place will reflect this absence of legitimacy.