Why the G3 design lacked useful growth margin

Was cancellation of the G3 battlecruisers a missed opportunity?

No; their limited refit margin made them less adaptable than a better-balanced contemporary battleship could have been.

The G3 battlecruiser combined impressive ideas, which explains the design's appeal. A better-balanced contemporary battleship would have served the Royal Navy more effectively. Major warships normally entered significant refits every five or six years, so a 1920s capital ship would probably be rebuilt several times before the Second World War. The G3 left too little growth margin for that cycle, making adaptability a strategic requirement.