Do you think slaves were happy?
Slaves aren't free to change jobs, so they aren't a counter example.
Feudal serfs aren't free to own land, so they aren't a counter example.
Or pre-unionized steel workers?
Steel was a monopolized industry (as are many natural resources), so steel workers aren't free to start their own steel industry, so they aren't a counter example.
Or the children working in textile factories?
Most of the countries you speak of, to start a factory you have to be friends with the local authorities, so there isn't freedom of competition there either. And an aside, we happen to have the highest (or close to it) teenage unemployment rate in the history of the world.
There are good counter examples to some common libertarian arguments, especially such as the scarcity of natural resources becoming more and more significant. However, the general libertarian argument that greater prosperity comes from greater freedom is still so historically well observed that counter-examples like the above fall apart after even the smallest amount of consideration.
With regard to this specific thread, placing more and more regulatory burdens on employers only serves to reduce the number of employers, which reduces the choices of the employees. Yes, it is possibly that there may be other artificial restrictions, such as the government-born monopolies that were mentioned above, on employers that make individual regulatory burdens less significant. But the better policy is to remove both obstructions, not to impose more.