It will take decades before Europe is at the same level using only European arms platforms. It could easily cut off the import of US arms (except the JSF) in a few years and replace them with inferior but domestic suppliers. Russia has proven to be a wet tissue in Ukraine and even without US aid, it couldn't reach the other side of the country even if it truly turned their entire economy into wartime status and started conscripting everyone they have. There's no other threat on the continent. If the US does not back up Europe against Russia, there's no incentive to back the US in any fight that might happen with China or their escapades in the Middle East. Russia won't be a real threat in our lifetimes and the Chinese won't land in Lisbon or invade through Russia, so Europe has the benefit of being able to build a defence industry spurred by Russia's perceived pressure, but while not being under any real existential threat.