With people learning about Fediverse, I'm guessing Instagram. Why? Because IG has become overwhelmed with influencers and TikTok-like junk. The initial wave of people into Pixelfed will be techies that want a break from that scene. Their friends will hear about it. The friends that just want to have an IG-like experience to share their pictures with friends while avoiding all the rest of the noise will slowly transfer over. Once they hit the critical mass, the switch will happen. This will take at least another year or two.
Yeah instagram is utter garbage. From photo quality to the current environment. Pixelfed is a really awesome federated alternative though I don't really see how it could resist the same fate if it ever got as much mass.
I think there's a missing link in your forecast: what will make people who are not techies, who currently use IG, stop using it, leaving behind their contacts at IG? They aren't going to want to use two platforms, so it'll have to be a clean break. I don't think hearing about alternatives from techies is going to do it, IMO. It's how a lot of people keep in contact.
Network effect is really sticky. Most of the users of the internet were once techie folk. Now it's everyone.