They can just operate and provide normal access to their services, just block AI access. This is already happening, apple would not release new Siri in EU (granted it's due to a regulation clash) but this would be a testing point.
If Europeans are still paying the same prices for sub par services/products to their American counter parts, it's win-win situation for those companies. Just sell a dumbed down non-AI version of service/product in EU for an inflated price.
A not-insignificant portion of the AI/ML research community is in the EU.
Regarding the open weights, I don’t see meta doing that for their future models, especially once they have their own frontier models. Open weight models are kinda marketing strategy where they use it as a bait and switch. A lot of Chinese companies became popular with their open wight models and once they build that reputation they have no incentive to keep on releasing new open weighted models
Also meta could just take the best mind in EU research centres to US with millions of dollars as yearly payments