There's a vast difference between America ceasing to support Ukraine and America supporting Russia. Simply never going to happen, that's a loonie scenario.
NATO is not officially supporting Ukraine. It just so happens that all the various NATO member countries are all individually deciding that it's in their interests to support Ukraine. If America drops out that's not going to change whether it's in the interests of those countries to continue supporting Ukraine. Indeed, it becomes all the more important for many of them to make sure Russia's strength is broken if they don't feel they can rely on America to support them.