> 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).