If retiring them was no guarantee that you'd be able to obtain replacement hulls that you wouldn't have otherwise, would you?
It's not just keeping the ships themselves in service, but the Dropshuttles too, when everyone else has gone to standard Dropships and docking collars (and all the other ship classes in your fleet use those), and the need to keep around maintenance facilities just for those idiosyncracies, and to keep training maintenance crews to service those features no other ship retains, and the need to be able to manufacture obsolete parts...etc., etc. Shipyards just wouldn't consider it worth the expense and headache...kind of like that story
Cray used to tell about the backup diesel engines on American subs.
So, despite the fact that Defenders may have had a prestige attached to them in the FSN much like that attached to Monsoons by the Terran/SLDF navy, at some point the FS admiralty should've come to realize that the supply and maintenance chain needed to keep those ships serviceable just wasn't worth it.
cheers,
Gabe