Can sweetened condensed milk be substituted for whole milk and sugar?
There are recipes that specifically use condensed and or evaporated milk as a shortcut to making fudge. It's not just a drop in replacement, it's a whole change in method. Just do a search for "easy fudge recipe" on Google to get examples of these fudge recipes that use condensed milk.

To add my opinion however, the old fashioned cooking and churning method produces the best fudge, but the condensed milk method although isn't as good at least produces a more consistent result. I do the old fashioned method. It's worth the extra effort and the extra learning curve (to recognize when the fudge is about to setup).