Not necessarily. A three or four year PhD in Europe is a short time to acquire a solid network (a lot of time is spent in training, unless the student is very proactive) and not all PhDs broaden their horizon to have a well differentiated topic to pursue after the degree. I would argue that true independence is actually acquired in the early postdoc years, but it requires a lot of work and a lot of luck.

TBH, 40% attrition rate is less I expected. Since each academic can train more than one PhD and postdoc every few years, which is the case, some attrition is required because the system cannot grow exponentially. The desired outcome in this context is that this talent incorporates to industry or other sectors where this expertise or problem solving skills are not wasted.