Is my dream of finding a light/commuter/hauling bike a pipe dream?
I think that it is generally true that any "all-purpose" tool is less-than-ideal for most purposes.  There are trade-offs for everything.  

If you are just looking for a bike that allows you to ride fast and long while carrying a load, though, I think you are looking for a light touring bike.  Get something with drop-bars and road-y geometry.  Get a comfortable saddle for it--maybe  Brooks B17.  It will go reasonably fast (not racing-bike fast, but certainly faster than my Globe Vienna). 

Personally, I am not following my own advice--I am doing what you suggest on a mid-80's Schwinn steel road bike.  I have racks and fenders on it.  It goes not-as-fast as my racing bike but fast enough for sure.  When I routinely carry lots of heavy loads on it, I start breaking spokes but that's the wheel's fault and not the frame.  The old steel Schwinns really do never break--if you put the right components on them, they will haul anything you want.  

At some point I may replace the Schwinn with something more modern.  After all, I'm running it with old 105 gear and down-tube shifters.  Generally fine, but when I can afford it I'll upgrade.  My Vienna has started to spoil me, with its IGH and shiny paint.  

In your position, I might consider a Civia Bryant.  Go with the belt-drive option.  It can haul a load, go fast, seems comfortable (having only looked at the geometry), and is low-maintenance.