Any player you add to your list is an investment. It isn't a given that Whitnall will play good football for us, and help us succeed - there's risk involved.
Assuming he is able to play AFL football to a decent standard, for how long? At what cost to the development of other players? What about the risk/reward of Whitnall compared to the risk/reward of a younger player who has a career ahead of him (as opposed to Whitnall)?
If successful, and that's in no way a given, Whitnall will give you a couple of years at best. No thanks.