>suddenly without rights after being solid contributors to that society, sometimes for decades.
This is a stretch by any measure. I'm an estern european in UK. Once I applied for ILR(Indefinite leave to remain) I can go and come with 0 problems. Not sure what "without rights" you are talking about but brexit doesn't bother me in any way.
We have plenty at work that arrived in uk around 2-3 years ago and they continue their lives without problems, go home for holidays and so on. Some work for us as self-empleyer/contractors. Not even those have any issues.