Closure isn't really what they're saying will happen though. They're just being realistic about the kind of contingencies that need to be created for a film studio whose core output is dependent on several retirement-age film makers.
Mind you, if Ghibli didn't have Miyazaki, would it really be Ghibli? It's not like they couldn't establish a Ghibli brand beyond the man, but given three decades, they've consistently failed to. Takahata disappeared, Kondo was a tragic loss and new talents were drafted in and lost with increasing desperation. Ultimately - and I say this as one of the site's most monotone ghibli lovers - it won't be the brand I cry for when Miyazaki stops making movies.