But the veto is basically meaningless, if the US leaves the UN the entire organization will be worthless.
No, and no.
Our UN veto means that the UN can't place global economic sanctions and embargoes onto Israel, for one thing.
For your other claim, keep in mind that many organizations survive today *without* U.S. membership; so too could the UN.
You would be making a bad gamble, with lives and entire economies at stake, if you played your hand such that the *only* way for you to win would be if the UN folded if we left.
I promise you that Europe, China, Africa, and the Middle-East would *love* to pay for a UN that had no U.S. veto.