Yeah, it's a tradeoff they have to make. A lot of "knowledge worker" unions decide to do this, hoping it's worth it for the institutional support they'll get. Some grad student unions were formed under United Auto Workers in recent years, oddly enough. I'm not an organizer, and I don't really have an informed opinion about whether it's worth it.

One hopes that these old unions could be revived with a bunch of new membership. What I described above is how they are now, not how they have to be.

Worth noting also that even unions with totally screwed up leadership often have passionate and talented organizers with a lot of experience and institutional know-how at the lower levels, just like tech companies.