Trams are not suitable as a trunk service from middle suburbs to the CBD - for example, in this instance it takes 50 minutes from West Maribyrnong to the CBD.
Trams have their place for shorter, local trips (say Ascot Vale to Highpoint in this instance), while a heavy rail service would be more suitable for longer trips say from Maribyrnong or Avondale Heights to the CBD.
As I said several posts above, the main users of a heavy rail service would be locals - not shoppers. ZH836301 is on the money with his comment - better tram and bus services linking it to other areas (ie Western Suburbs) would bring in far more shoppers.
What's wrong with making the '82 a through running cervice to Melbourne CBD?
And what exactly would the point of that be? Not everything needs to be centered around the CBD - and it would remove a major destination (Moonee Ponds) from that route, which exists purely for suburban trips.