It's a fair point to say that the counterfactual is hard to draw up.
I will point out the main reason it's hard to come up with, is the fact that American aid, weapons sales, and diplomatic support for Israel has been so constant and unchallenged over the past several decades that we don't have many good examples of what Israel would do without impunity.
Sure, but Israel has also been continuously attacked so we're not sure how Israel would act if it weren't continually being attacked. The counterfactual is hard but it's not hard just because the United States has supported Israel, but also because other nations in the region have always attacked Israel and continue to do so.
I think there's an inclination on the Internet to lean toward one side or another, but just like with Democrats and Republicans, at least in my view, everyone sucks here.
I will point out the main reason it's hard to come up with, is the fact that American aid, weapons sales, and diplomatic support for Israel has been so constant and unchallenged over the past several decades that we don't have many good examples of what Israel would do without impunity.