Rosyth is a privately owned dock yard, its not a Naval Base so its not up to the MoD to close it, its Babcock's responsibility. However, believe me it is the only dockyard capable of doing military refits that can take the QE class. it has all been looked at. Ark Royal was smaller than the QE, not by much but by enough to a difference.
Note, Devonport & Portspouth Dockyard are MoD owned sites with seperate Naval Bases within them. Faslane is a Naval Base & not a dockyard.
As I said, above, we should close one of the dockyard/ naval bases but which one? I vote for Portsmouth & move the RN surface fleet to Guzz but it would need a short term investment & we don't have the cash. I genuinely don't believe that the closure of Plymouth airport will have any bearing on the future of Devonport.