That depends on the situation of the person being claimed for I would imagine. Some conditions, such as severe congenital disabilities, would never improve and once a claim has been established there'd be no reason to periodically reassess that individual. Other conditions show patterns of improvement and deterioration and I would expect claims relating to those to be monitored and periodically reviewed.
Many of the cases you see where the claimant is complaining about regular reviews or is caught out relate to conditions that are very difficult to assess objectively, eg "bad backs", ME and fibromyalgia, which can be exaggerated by the claimant.
In Margaret's case I would have expected social services/DWP or whatever to carry out periodic reviews, as though her learning difficulties were obviously present since birth she had been attending various classes to help prepare her for more independent living so her situation was likely to change over the years.