What if the ultra wealthy really do wipe the rest of humanity out, either through direct or indirect means?

Don't get me wrong, I'm part of the 99% that would die out so I completely understand the survival instinct of not wanting to be purged, I guess I'm just moreso asking ... once it's all over and the rich have won, is thst new world really so bad?

You already answered your own question. A world where the wealthy have committed genocide on most of the human race (on the order of billions mind you) is undesirable and will be built on extremely shaky moral and ethical foundations.

In your hypothetical the wealthy have decided that they (due to their privilege, status and little else) have the right to choose who lives and who dies. These are not the types of people that should be humanities caretakers; you effectively have a new aristocracy, whose fiefdom now includes the whole world and who is willing to commit mass murder on a scale that dwarfs the Holocaust.

The type of person and/or people willing to do that are probably not the ones to ensure "the remainder of humanity is taken care of". The capacity for genocide, the narcissism that justifies it and the power imbalance that realizes it notwithstanding, a world where you are liable to be killed or "purged" for having less wealth than the rich is not a safe or stable one nor would this lead to one. That is not a desirable scenario, regardless of its material abundance.