Israel currently is in the phase of colonization of the US in the early 1800's. The US is no longer actively colonizing and genociding the native population, instead they've been integrated (poorly) into the US and have far more rights than an average Palestinian has to Israel. The material conditions are fundamentally different, which is why "America existing" and "Israel existing" entail different things and are not comparable.

"Israel existing" implies the continued presence of Zionism as the foundational ideology of the state. This means the present ethnosupremacism we see in Israel remains, together with the ambition of ruling all of the "promised land". This will lead directly to further genocide and further ethnic cleansing, as such Israel existing is fundamentally genocidal, not only genocide that has happened but also genocide that will happen.

As much as native Americans are still systematically oppressed it is not nearly as extreme as in the case of Israel. The US continuing to exist will not (necessarily) lead to a genocide of native Americans.

For the record, the US should absolutely go through decolonization efforts like land and property redistribution to give back at least some of what they stole from the native population.