Why the Emerald class could not simply be redesigned

Why did Britain not redesign the Emerald class around all-or-nothing protection?

They were too far along to redesign cheaply; completion, abandonment, or a carrier conversion were the realistic choices.

The ships were already too far advanced for a clean all-or-nothing redesign. Britain could finish and modify them, discard the work already done, or pursue a fundamentally different conversion. An aircraft-carrier conversion with deck-edge lifts and a modest armored deck is the preferred alternative. Construction progress can close off otherwise attractive design choices.