One of the challenges of this proposal is imagining a future world where users understand this concept and then figuring out how to get from the world we're in today to that future state. One way to do that is to have a more specific conversation around what the user interface might look like for First Party Sets/Site Groups. I am far from a designer but mocked up the attached example (modifying the default Firefox UI) as to how this concept could be explained to users.

With an interface like this, the FPS/SG name would always be exposed in the URL bar so the user can always see the domain they are on, but also the set/group that it belongs to. If that element became standard across browsers, this would become an easier concept for users to understand. The idea would be that you could click on (or mouse-over) the group name and get more information about what this meant and how you can interact with it. This interface would need to be simplified for mobile browsers due to space constraints, but perhaps an icon similar to the secure "padlock" can be created to show that this site is part of a group and to tap it to get more information.

In the early days of the web, there was significant confusion about how to type "HTTP://", when you needed to put "www." in front of a domain, and what a domain name even was. But over time, through education, reinforcement and repeated usage, users understood how the concept of site names/domains works - generally speaking. If all browsers adopted this feature and standardized on the interface (relatively), the same process would happen here.

The key question is what are the true user benefits that can be offered via sets/groups - and then if those have real value, we can collectively figure out a path to get there. But it's certainly true that to push the web platform forward, sometimes users will be confused about new features, but they will learn them over time.