Unless she suffers severe cognitive decline, her education will always be useful, particularly for the life that she will be leading! I should think her degree will equip her well for conversing with crusties at the thousands of meals she will be expected to attend and at which she will be able to display a high level of cultural interest in the surrounding art. Academically, she is more intelligent than Diana was - she achieved 2 As and a B and a 2.1 from a well respected university, although in an age of grade inflation, that may not be as of an achievement as it would have been in Diana's day.
Whether she will have the perceptive intelligence that made others warm to Diana, especially when visiting victims in mundane or hostile surroundings, remains to be seen.
I know that some people expect that a degree should lead to career progression, with each successive job being of higher status and salary than the one before as they get older however I don't see anything wrong in learning for learning's sake. If money isn't a consideration, then a more lateral experience of studying another degree at a later point in life, taking different jobs that are taken just because they are of interest or in Catherine's case, helping her family business, isn't any less respectful.
Those with money always have and always will have greater choice.
Over the forthcoming decades, she won't be able to spend much time with her family and a parent can sometimes die earlier than expected - she may be glad for the time spent with them while she had the opportunity. Besides, many know of someone who qualified as a doctor or barrister and then pursued a different path; that is just as, if not more annoying. She has achieved her goal, I suppose, as her time will be dictated by a public & social agenda from now on - a full time job in itself.
In some ways, she's a sacrificial lamb. Her siblings will have elevated status and far more fun by dint of their sister doing 'time' and the family business will, no doubt, go from strength to strength because of the connection! Catherine will have little time, truly alone, with Will - imagine wanting to say "What the -fuck- heck did you say that for?" and knowing that all the flunkies are watching/listening or the long lens of a paparazzo catching a glum look or 40 something jowls? I wouldn't want that. It will become even harder as she ages and she the comparisons between her and how her deceased MIL looked at her age not cease.
So, no, she shouldn't have had to work anywhere other than for her family's business because her desired position limited her freedom and choice.