Part of any international sanctions is to leave something for the perpetrator to lose.
Otherwise, they can do literally everything without any further consequences whatsoever - it won’t get worse for them.
Also, as rightfully mentioned, part of UN’s goal is restoring peace between nations, which is harder to do when they are not members. That’s the problem with Palestine, and it will get worse if Israel leaves too.
The entire article is utter apologetic trash, doing its absolute best to show how unpopular this decision is (despite being hugely popular) and focusing on “Hamas terrorism concerns” without any consideration at all given to Palestinians.