Pre 1964 the luxury of choosing where you lived was largely reserved for the rich, who had time and money to organise their integration. There were fewer concerns over security, abuse of social security benefits and so on which has created some of the administrative requirements.
For the poor it was another story. People went where work was, and that was the springboard but with many constraints. Or they were refugees and moving countries was not easy! My grandparents can attest to that!
You are referring to people moving here while the UK was in the EU and had the fundamental right to live and work here. Ok some didn’t follow the rules but we had to do a ton of paperwork when we arrived. And absolutely no english speaking help available, or internet forums.
I agree completely with @JaneJones about the migration situation pre EU membership for UK citizens. This is a situation which is now returning, in that only the moneyed can realistically change country, apart from refugees, whose situation is not enviable in any sense!
Moving pre-Bexit was relatively straightforward as there were no visa requirements for other EU countries (of which the UK was one). As Jane points out, however, there were (and are) reams of other paperwork to complete in order to become part of “the system” here in France.
In terms of the very frustrating hoops non-EU nationals have to jump through to obtain residency, there are quite a few blogs on line, going back years, of Americans trying to move to France! Same for all other non-EU countries. The only thing that has changed is that the UK is now one of these countries when it was used to the ease of being an EU one. Quite a shock in very many ways,
Added: Some, but not all, of the communication problems you are experiencing here @kazz4662 are due to the pandemic. Prior to that,it was relatively easy to walk into your Prefecture or CPAM office and explain your problem. Those of us applying for CdS etc now but who have been here for years are also hitting these frustrations so can readily sympathise 