I think those things are very hard to predict. Yes, many Europeans will stop working for American companies and lose their very high paying jobs. On the other hand, the EU as a whole will stop sending billions of euros to the American economy, and at least some of this money will be invested in creating local alternatives; Those who worked for American companies will probably find their place in these alternatives.
Everything you wrote about the open market system is true, except it seems like that system have died over the past year. Europeans understand now that the US isn't a friend.
My worry there isn't (just) Europeans who directly work for American companies but Europeans who work in the _markets_ that US companies created.
For example almost all digital marketing agencies are almost pure resellers of big tech products: ads on Facebook, Google or Microsoft networks. Similarly majority backend solutions are delivered over 3 big cloud providers' specialized products like AWS Lambda or GCP BigQuery. The definition of Mobile Software developer is someone who writes software for Google or Apple platforms. The entire fields will disappear overnight.