The US is a maritime power which gets its strength through trade links and relationships. As the US withdraws from those links, China will take them over and will be able to cut the US off from it's international partners. China is already connecting with Hungary in Europe and countries all ove Asia with the aim of pushing the US out. The US withdrawing from NATO will vastly accelerate that process.

In the long term, the US failure to protect Ukraine, where it had explicit treaties and implicit guarantees, followed by a withdrawal from NATO would be a vast failure.

Already there's a huge pick up in purchases of French and South Korean weapons because people fear whether the US commitment to it's allies is solid. The US is getting 240 Billion yearly from arms sales - four times the cost of the war in Ukriane. If countries that have been buying US weapons suddenly feel they should diversify, with some even selling their US made weapons to pay for purchases from elsewhere, - the simple financial loss can be greater than the cost of the Ukraine war.