I think I disagree. I have heard this a lot on Reddit and I've heard it about Twitter, Google Plus and a bunch of other social networks and I've been on small ones and huge ones alike. Honestly, to me, when a social network is large it includes both nuanced discussion and there more casual posting. I don't see why both can't exist on the same site and I feel like it often does exist on the same site.
I also think people have a huge range of interests, some of which might be quite niche and having a large user base means these niche communities can thrive. When I've used smaller social networks, this typically has been the problem. They often have their tech communities covered and they often have other large common hobbies and interests covered, but if you take for example learning welsh or theremin music or something else, then you typically only get communities about those things on larger networks.
Reddit is nothing without users posting and upvoting posts and comments. If all, or a large proportion of the users stopped using the site, reddit would have to listen or they'd stop being useful. I think there are two problems:
As you said, users don't realize the power they have. It's a bit more nuanced than that, they do realize the power of the collective, but don't think the collective will exercise that power, and thus won't act individually. It's the same as "my vote doesn't matter, it's just one vote". This is obviously a self-fulfilling prophecy because they are making it happen, they simply need to follow what they think is right.
A lot of users don't care. Again, a bit more nuanced than that, most users probably have a preference reddit listens to their users, keeps the 3rd party app access, etc. But they don't care enough to do anything about it, which in effect means in any practical way, they don't care. I'm guessing that to them this feels a bit of a "niche" problem and will use the official app. There are a small amount of users, like me and probably you reading this who've left reddit and won't go back.
The protests have worked. They've moved a motivated minority over to lemmy and we're creating communities, posts and comments, contributing to apps and running instances. We'll spend our time and effort improving the tools and communities for the fediverse ready. Hopefully, with enough of reddit being reddit causing more waves of people in the future to seek another platform, the fediverse will grow and reddit will dwindle. That's my hope anyway.