I wouldn't replace protein powder with skim milk. 1 glass of skim milk is less than 10 grams of protein. Some people are lactose intolerant. Milk isn't portable (IE. Kept in the gym bag all day until your work out is done).
I could mix my protein isolate into less than half a cup of water and drink it for 35G of protein and 130 cals. In addition, protein isolates are already broken down and the body can absorb it more readily
This nonsense about "isolates are already broken down and the body can absorb it more readily" is a pile of BS. To properly absorb protein you need the right factors present, various vitamins and minerals to go along with the protein, to make it usable.
Guess what milk has in pretty much exactly the right proportions? To replicate the vitamins and minerals that milk delivers along with the protein you'd have to go get a science degree and work out the right proportion of artificial vitamins to go along with your artificial protein. Maybe you will get it right. Maybe not. Or you could just drink milk, which is a hell of a lot cheaper anyway.
You do know that milk is the product of a million year process of evolution and nature resolving the basic problem, "how to bulk up and grow as fast as possible", i.e., what growing babies and children do, doubling or even tripling their body mass in a year by drinking milk. It's pretty much the ideal "bulk up fast" food, and your body was made for it. (Well, the human version, but the cow version is pretty close.)
And why go all NASA and try and condense it all into a delivery system with less water weight? Are you worried about lifting the extra water weight into orbit? Last time I checked you sweat during a workout, it's not like you don't need the liquids!
OK--if you are working out in the jungle far from any refrigeration system, or on the top of some mountain, the fact that you have to keep your milk cool might be a problem, it might be easier to pack in your protein powder in your back pack--but I am guessing you'd have a bit more difficulty packing in your free weights. Most people don't have too much trouble finding a fridge or a convenience store.