As I don’t know what diagnoses SIL has, it’s difficult to talk about, but imo it can be unrealistic to talk about independence. What it really leads to is self neglect.
For instance, DD has come across patients who are sectioned for being psychotic in public places. They may go to sleep in trees and on pavements, despite having a council flat. Once in an inpatient unit, they get regular food, drinks, meds, and sleep, through living in a routine, where other people take care of them, by doing the shopping, making the meals, keeping the place clean, talking to them, etc. Once they are stable again on the meds, they go back to their council flat.
There they don’t eat properly, may only drink one lunchbox juice carton a day, never do their hair (so it ends up matted), and don’t take their meds. They are socially isolated and spend their time on their own - hence the sleeping on pavements to be among people. They end up being sectioned again, but this time, they lose their clothes, their laptop, etc in the inpatient unit.
These may be intelligent people, such as graduates.
They are not assessed by social workers as needing care, and are not even under the CMHT for mental health support between admissions. All they get is a council flat and benefits.
DD walked out of her job two and a half years ago, and basically since then, she has lazed about in bed all day, playing on her phone and is then up all night. She doesn’t do any housework, shopping, exercise, cooking, you name it - I think she does wash her clothes. She can do these things (she’s very bright, but cannot organise herself to save her life) but won’t citing chronic fatigue. Her OH works full time, does the cooking, the life admin and they go shopping together at weekends.
I don’t know what future planning you expect PILs to do? Nobody has ever offered us any help in planning for DD long term. I have no idea how long single people with severe mental health conditions are on the waiting list for a council flat, but here LHA won’t pay enough for a room in a shared house.