Let me put it this way -
we have enough holes to fill as it is, if we need to find a new RUNNING BACK on top of everything else, you can kiss next season goodbye. Whether you like it or not he's unarguably better than over half of the starting running backs in the NFL, and he could very well be top 10 if given a chance. Getting rid of McGahee and devoting our time to finding a replacement who will more than likely be worse than McGahee makes no sense whatsoever.
Take a look at free agency. Do you see any running backs that could be a decent starter? Nope.
Now let's look at the draft. Only one of the top few backs would be a safe bet coming into the season and even then so the only back who I think would be any sort of upgrade over McGahee is Peterson - but if we take Peterson, there goes our first round pick, and besides we'd have to trade up to get him, so say goodbye to our second round pick as well.
So let's throw away our first and second round picks so we can draft an unproven running back who's never played in the NFL before and may be worse than McGahee nevermind as good or better.
Now do you see why getting rid of McGahee right now makes no sense whatsoever?