A good solid DM can shield all the back four and cover when needed, he can also see where the danger is going to come from and act on it. There are other strengths to having a DM ( just think back to the days of Wanyama, Wilson, Roberts ( i know he also played CB) and see how they made us look more compact.
I am not suggesting that our FBs are the best around but our defence is over run on most occasions because we lack a player who dominates in front of them. But i accept that you think upgrading our FBs will solve some of the problem but just look at the sides that have been strong over the years and they all have a very good DM in front of the back, we do not and it shows.
Stop! Hammer time.