Regarding your first point: if your immediate next-of-kin is unable to take care of things, do you trust whoever is next-nearest to respect your wishes? How confident are you that this will still be the case after time (t) has passed?
My experience with a few friends and relatives: often such informal estate planning is done as a one-off, in a moment of forward thinking, and then left as is, sometimes for years. Unexpected changes in the views of the next of kin (in my family's case, various religious conversions) led to all kinds of family disputes related to different opinions about what the desires and intentions of the deceased might have been - and whether they are even relevant if none were formally expressed.
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