Sorry, just wading into what seems to me a very interesting thread (have just read the entirety).
It seems to me that as individuals, we all have different levels of need, and the onus is on us to express that need as and when we feel it.
People cannot be expected to mindread, or 'assess' the levels of needs of others because it just isn't accurately possible, and opens up all kinds of unpleasant avenues of assumption.
Is a person pregnant or overweight? Is a person 'disabled and in need', or 'disabled and angry at being patronised'? Is a person 'elderly and fragile' or 'elderly and full of vitality, annoyed at being 'labelled''?
People may try and judge the needs of others in the most helpful, friendly way and be soundly criticised for this as 'discriminating', 'assuming', etc. And these points too are valid!
Which brings us back to the point that the only person who actually understands the level of need involved, is the person experiencing it. And so yes, the onus is on them to communicate this in some way (hopefully politely).
This then bounces the ball into the court of the person sitting who has to ask themselves honestly whether they are well enough to stand, and if not, to express their own level of need back (hopefully with empathy and politeness).