One thing a VPN does is prevent your ISP from selling your browsing data to third parties. If you have Comcast or Xfinity it's worth it just to deny them even a penny.

This is true, but you also gotta consider most people do browse and go to other websites than just ones they log-in to or social medias. I think using a VPN generally makes it harder for other websites (like news articles as an example) to track you across the web. (For instance, if I visit Website A with unique IP Address Y, and also visit Website B with unique IP Address Y, even without logging in or directly giving them any data, they could correlate those 2 things. That's where I think a VPN can really help things because it gives you a large pool of users in this case without using your unique IP).

Even besides this, you're missing another point. I'd argue the largest benefit to VPNs is just preventing your ISP from collecting and selling the websites you visit and metadata around them. That's a huge and undeniable benefit to using VPNs for privacy if you use a trustworthy and reputable one, just being able to prevent your ISP from seeing what you're doing, when you're doing it, etc, which is especially important with how dodgy ISPs are and how most collect and sell user data.