What do you mean by source separation being medicalist? Well-meaning question, sorry if we're imposing by asking!
It assumes a medical view of fictivity that doesn't inherently apply to all fictives. Not all fictives are introjects and the original use of fictive was for people who experienced characters coming to life, usually through writing, roleplay, etc, which would usually involve them being 100% their source. Some fictives are more spiritual or walk-ins. The framework doesn't work when applied to non-introjects who might have reasons for just being their source.
Even with introjects, I know there are some who reject the idea that they're just "based on" a fictional character and might just consider themselves to be that character. All types of fictives can have deep views on their identity.
This term was also coined by fictionkin, which is another framework some fictives may fit better and likewise "source separation" isn't really a concept in the same way in fictionkin communities because it's recognized as an involuntary identity.
It also doesn't feel like it accounts for natural character development. If you exist in a system as a fictive for decades, of course you're going to be a different person, but you can still identify as your source and be just "source but 10 years have passed that I've been in this body".
There is something to be said about avoiding harmful behaviors based on your source and finding hobbies outside of your existence in a system in general. I know there are also issues with fictionfolk being too possessive of their source, which is where I've seen a lot of people assume fictionality is unhealthy.
The issue is the way source separation implies you can't be a fictional character and be "functional" is a saneist view that also goes against the ones who coined source as it's being used here, which were fictionkin. There are better ways to encourage developing yourself as a headmate and being less possessive over others - the later I think is a much more important conversation right now because of doubles discourse coming back.
While this isn't a subject we can speak easily about (factives can trigger depersonalization and so we usually just avoid discussing it in depth), I also don't think this should inherently apply to factives. While I don't think factives and fictives are compatible (i.e. telling a person you are them is very different from telling a person you are their character), I don't think telling people "just stop identifying as them" is helpful and it's better to discuss actions and consequences than something that's out of their control.
The same goes for genuinely problematic identities. I think it's better to discuss the ways you present those identities and find a healthy balance that works for yourself than it is to force people to stop what is generally an involuntary identity experience. I don't think any of these should be censored and there should still be spaces to discuss them.
I hesitate to call it inherently medicalist, because there are absolutely cases where some fictionfolk might benefit to stepping back from their identity and that's not inherently medical, but the way it's presented in the plural community comes across as medicalist and I think the premise as I've seen it used is medicalist.
