What is the solution? Make unions mandatory?

The solution is winning hearts and minds to convince workers to organize. In the past, corporate propaganda and improving living conditions convinced many that unions were a net negative. As conditions change, so too can those convictions.

But I don't think mandatory unions are a solution. Unions would have to be very popular for such legislation to pass. But if they were so popular, they would no longer need compulsory membership.

In general it is unions and organized labor that push legislation meant to help workers, so starting with legislation instead is putting the cart before the horse.