Well, an important distinction in Hollywood versus my understanding of most other unions is that being a part of a guild does not guarantee you a job (though it is a prerequisite for many of them):

You still need to be hired by a production, who chooses to work with you and sets your rate.

The guild just provides a guarantee of pay floor, hours, conditions, healthcare, etc across and transcending individual companies.

So the guilds could theoretically let in as many people as they wanted without diluting supply and demand, because membership does not mean actively working.

The Guild system is unique because its as much as a credentialing system as a union, which is why I think it works.