Saintly was placed in the Metrop over 2400 ( or was it 2600? ) TWO starts before winning the Cup; the start before the Cox Plate. All of Bart's ten winners since Light Fingers have had a 2400+ in the lead up.
M & P, Saintly, Let's Elope were 4YO's when they won the Cup. As 5YO's they would have received more weight. This WFA comparison that I talk of is not my opinion it is a real consideration of all handicappers and ratings people when doing weights. Horses improve as they get older - even from 4 to 5. The WFA scale shows that and it is relevant when weighting handicaps.
I don't know whether Northerly would win with 58 or not but I'm pretty sure we won't find out because Kersley won't have a go. Saintly would probably have won his year with 57 or 57.5 such was the ease of the victory. Given that that being a 4YO that is WFA and Northerly is under WFA, I do not think he would have "no chance". He would have some chance.
You ask me to compare Northerly with the champs you mentioned. I agree that he would struggle on the weights against all you listed. However, there seems to be a fall in quality of Cup fields since the great fields of the early to mid 90's. If you had asked me if Northerly could beat Central Park with 57 or Jezabeel or Brew or Rogan Josh I'd say a definate maybe.
For mine I think Bart has his strongest team since the early 90's. I am a big Magical Miss fan and I think Benarrivo is his Cup smoky if it gets a run at 49kgs. It's a shame that the WFA stars won't be there to test them out.