The lack of anyone 'big' coming out of Ghibli gets brought up alot and is seen as a failing of Miyazaki (semi arguably since Anno is plenty big) but I personally don't see it as being a big deal. Maybe the reason Miyazaki can make movies like Wind Rises and Spirited Away is because of his crazy attention to detail. I'd rather have those movies than some watered down movie made for the sake of building up staff.
When Takahata finally announces his retirement, it will be the effective end of the Ghibli we know and I choose to look at their work to date rather than fretting about a nebulous lack of a future.
I don't know how Studio Ghibli is run but I have consumed bits and pieces of information with regards to their organization from what others have said and experienced there. Considering that Miyazaki has what amounts to basically supreme authority with regards to the working of the Studio I think it's only fair that he, and others, should share some of the responsibility for failing to nurture creative talent at the studio.
I don't know if it's a question of either having Miyazaki's great movies or helping to nurture new talent - surely the studio could have tried harder to give younger people some projects that they could work on with some independence? I understand this would be difficult considering how important Ghibli brand name is and how it needs to be associated only with quality works but I certainly believe they'd had the time to work around this issue.
Moreover some of the more high profile problems relating to Miyazaki's management practices are well known - the (presumed) firing of Mamoru Hosoda while he working on
Howl's Moving Castle which led to a rather poor Miyazaki film and the exponential rise of one of the newest talents in the industry. Then there's the nepotism which led Miyazkai to assign his own incompetent (in terms of directorial ability) son to direct a movie when surely there were more talented people available, and then to have him direct a second one later on. None of these seem like wise moves for the health of the studio and I think it's fair to highlight these issues when considering his legacy overall.
With regards to your second comment, I don't want to solely harp on about the perceived lack of a future for the studio that Miyazaki, Takahata and others helped to create, rather it's just one of many different topics that I considered worth bringing up with regards to the announcement of Miyazaki's retirement.