It is difficult paint everyone with the same brush.
It's true that Dukies take a bit more time to adjust to the daily grind of clinical work. But beyond the initial years, there are a fair number who are just as competent as clinicians.
Furthermore, being a good House Officer / Medical Officer is but the first step into the medical career. Dukies have their own background (e.g. engineering degree, business experience) and interests which carry them further as well into different realms of clinical administration / leadership / research / business.
Of course, some would have a change in priorities as they are older and doing the same work as someone who is 10 years younger, being on call 2-3 days per week at 30+ years old is certainly no mean feat. This group may choose to be GPs as a lifestyle choice.