That's not gonna be a very successful union then.
The unions I know that actually work are near-universal, but you're never technically required to join.
However, at certain levels of unionization, peer pressure kicks in because the "freeriders" effectively get protection from the union without pulling their weight.
Their peers have the same material interests to keep wages low in order to keep profits high, so they rarely side with the workers. There are many documented cases of explicit collusion, even.
It's why it's important for workers to act collectively. Since they can threaten the owners' profits through strike, there is much more leverage.
I'm wary of centralized power of any kind, but explicitly coordinated centralized power (in the form of industry-wide unions) scares me just as much as explicitly-illegal-to-coordinate-but-aligned-incentives power in the form of a set of a few gigantic employers.
And yet the unions are democratic, but the employers are not. You can vote out the union leadership and all important decisions require a ballot.
Power structures can be abused, but they can also be a way to concentrate the will of many to achieve something that cannot be done individually.
What's important is the material incentives of those involved. Even if they were to be insincere (in which case they often are removed), union leadership are incentivised to raise the wages of all members since dues are the main way they are funded. Employers are always incentivised to reduce wages as much as they can get away with, since they wish to maximise profits.
