There always has to be one that ruins anniversary episodes by not wanting to come back. Capaldi being a "huge fan" of the series should know better by what Tom Baker did (and has regretted since) by not ing 'The Five Doctor's'. That's okay though I won't anything Capaldi does beyond Doctor Who either.

It wasn't the producers mistake. As said in Lis Sladen's book, she (as Sarah) was often brushed aside when it came to Harry. Lis was going to quit unless changes were made. Harry was then written out. Sarah then went on to become the most popular companion.