Type 21 could not wait for Type 31 or Type 32

Would surviving Type 21 frigates have been replaced by the Type 31 or Type 32?

No. The Type 21 was not designed for such a long service life and would have required an earlier replacement generation.

Keeping a class in service on paper does not make its structure, machinery, and combat system last indefinitely. The Type 21 would have reached the end of a realistic life before the Type 31 and Type 32 programmes could replace it. Britain would therefore have needed another frigate solution in between. The passage is useful for thinking about force planning as a continuous sequence rather than a list of today's class names.