I hear you but I think you are simply asking for an entirely different blog post. I don't think Verity's aim here is to give an introduction to `Selective`, but rather to introduce a formalization for it; something which has been notably missing for those who think about these sorts of things.
I understand the original Selective Functor, so an introduction to that is not what I'm after. I want to understand this new formalization, because it's the kind of thing I use, but I'm not a theoretician. If the goal of this post is simply to explain the formalization to the small number of people who are already deep into (category) theory, I guess it does a fine job. However, I think a better post would be more accessible.
I think the blog post does a good job describing the idea of Selective ("finite-case" etc.) but for me it falls apart shortly afterwards. If I was writing it, from what I understood I would start with the overview, then describe `CaseTree`, and then go into what abstractions this is an instance of.
As a small example of how I think the writing could be improved, take this sentence:
"This is in contrast to applicative functors, which have no “arrow of time”: their structure can be dualized to run effects in reverse because it has no control flow required by the interface."
This uses jargon where it's not necessary. There is no need to mention duality, and the "arrow of time" isn't very helpful unless you've had some fairly specific education. I feel it's sufficient to say that applicatives don't represent any particular control-flow and therefore can be run in any order.
