This seems like a leap of logic. I see there are only two possibilities for that to hold true.
a) There are no illegitimate states
b) Israel is unique and unquestionable
I don’t think (b) is worth discussing as such exceptionalism is at the base of all genocidal states, and are almost always illegitimate (I’ll make an exception for the Vatican)
And (a) is easily disproven with a simple look at history. There are plenty of illegitimate states throughout history. A recent example is Apartheid South Africa which got replaced by a democratic South Africa in 1994. A more blatant example is Rhodesia. Almost everybody questioned the legitimacy of Rhodesia, and any anti-Rhodesian argument was indeed rational. Rhodesia was funded as a racist, white-supremacist, ethno-state and as such had no right to exist. Most of the world agreed. Not even Israel recognized Rhodesia (despite militarily supporting the white ethno-fascist government).
Israel in its current form has not right to exist. Israel needs to either a) grant full sovereignty to Palestine, and either pay reparations or grant the right of return to the descendants of those displaced in the Nakba and following atrocities; or b) unite with Palestine under a single democratic government with equal rights of all citizens. It cannot exist as a Jewish ethno-national ethno-state, which subjugates Palestinians in a settler colonial fashion, and further antagonizes its neighbors who shelter Palestinian refugees with constant military incursions.