How surviving Neptune cruisers could preserve the six-inch gun

Could surviving Neptune-class guns lead to Vulcano-like six-inch ammunition and a longer-lived British heavy-gun capability?

Yes; keeping the calibre useful could support new ammunition and eventually put six-inch guns on later destroyers.

A long-serving Neptune gun system could attract guided or extended-range ammunition similar in concept to modern Vulcano rounds. Continued operational value would create a reason to preserve a six-inch manufacturing and support base. After the Falklands War renewed interest in naval gunfire, Britain would then face a choice between building new cruisers or fitting the heavier weapon to destroyers. In this alternate development path, postwar British destroyers might carry six-inch guns rather than returning only to smaller calibres. Keeping one capability alive can reshape later ammunition research and ship classification.