Those legal protections are easily eroded without unions.

Just look at Finland. Here, the current government first made it illegal for unions to strike when the government takes action to weaken employment law, and then they significantly weakened employment law.

The only protection left is collective bargaining agreements, which can still uphold some of the old legal protections through contract law.

This was also only possible after decades of work by industry lobby groups to significantly weaken unions by targeting them with tax code changes, splitting up unemployment funds from unions (with the employers then founding their own unemployment fund, so that union membership is drained).

Unions are the only defense that workers have. If there are no unions, the employer can have their pick among desperate job seekers, and give them the lowest wage they can live on.

> Unions are the only defense that workers have. If there are no unions, the employer can have their pick among desperate job seekers, and give them the lowest wage they can live on.

The major other defense is competition. If there were infinitely many workers then unions would be useless to begin with, because employers would just let them walk out and hire someone else.

Individuals can likewise use the fact that labor is finite to make employers offer more by credibly threatening to work somewhere else. This is trivially shown by all the people who make an above-median wage despite not being in a union.

You do, however, need competition for that, and in turn to not have laws that prop up incumbents and create barriers to entry to new companies (i.e. new prospective employers).

> Those legal protections are easily eroded without unions.

That's very clearly not true based on the situation in Western Europe, in fact legal rights tend to keep increasing even when union membership is decreasing (e.g. UK, France).

As said, jobs and society as a whole have evolved and noone can be elected in government by promising to take away important protections, what they can be elected on is promising to curb union power but that the unions' fault when they abuse striking action.

My understanding is that workers are extremely well protected in Finland and what's happened is only some restrictions on political and solidarity strikes.

Edit: Unions are not the only defense that workers have. We have democracy with all sides represented and nowadays (in Europe at least) more is done through elections than through unions. That's why I said that society had changed compared to the early days of the labour movement.