Watching season 12 recently it's interesting how Harry is used. I know he was originally intended as an Ian Chesterton or Steven Taylor type, being the more physically active member of the cast while the Doctor was intended to be a Hartnell-type.
Instead he's kind of more of a kind of opposite to Sarah in terms of his political views in some dialogue exchanges, often is the butt of the Doctor's jokes (Sort of like a proto-Mickey) and instead of being the physically active one he's falling down, causing a rockfall or getting his foot stuck in a giant clam instead. He really shines in Genesis of the Daleks I think where he spends most of the story with the Doctor without Sarah though.
I think we lost Ian way too young, he of course co-wrote the Scratchman screenplay and several Target novels. Also did some work for Disney too.
Kind of wonder what a "What if" season 12 would've been with say, an older Doctor.
Seems kind of unlikely that there would be a similar situation in the future, with an older actor taking over and the companion doing more of the heavy lifting with the action. Capaldi for example was the same age of Hartnell but much more healthy and could do a lot of action stuff. Colin Baker of course was intended to reflect Hartnell's grumpiness but was very physical active in his stories.
Ace did do a lot of the action in the McCoy stories though so I guess that might count as a similar situation classic-wise.