First of all, Europe, apart from the UK, has not signed the Budapest Memorandum:
en.m.wikipedia.org
Let me tell you what would happen if the USA stopped supporting Ukraine.
After the fvckup in Afghanistan, and a potential fvckup in Ukraine, many countries would conclude that USA is a joke protector and they would turn to China. Before that, China would invade Taiwan and the USA would lose access to the most advanced microchips. As a result, the USA would lose power in favour of China and would be reduced to a regional power.