Why Eagle and Ark Royal could not serve forever

Should Britain have retained and continually modernized Eagle and Ark Royal instead of building replacement carriers?

No; heavily used ships accumulate structural and saltwater damage, so modernization cannot substitute indefinitely for planned replacement.

Large carriers cannot be kept indefinitely simply because their formal commission dates seem recent. Eagle and Ark Royal embodied older design origins and had already endured hard operating lives. Saltwater corrosion and accumulated structural wear eventually make another modernization less economical and less certain. Delaying replacement construction transfers the risk to the fleet, as aging Type 23 frigates and late Type 26 deliveries illustrate in a later generation.