BB doesn't spend money foolishly. He went into 2016 with what he thought was a good group of running backs. However, Blount wore down and once again wasn't effective for the playoffs and is a free agent, and Lewis never effectively came back, and when he did come back he got injured again during the Superbowl. James White came on strong in the superbowl and maybe a piece of the solution, but the Patriots want multiple good backs.
BB decided to move on from Blount and at least have a second option, who by the way is a great special teams player, for Lewis in case he never comes back, or continues to be carry restricted.
I am not sure if this qualifies as a change in philosophy quite as much as a change in personnel. But either way, the current market for running backs being what it is, the Patriots can get two backs who by some metrics are among the top young backs in the league for a 5th round draft pick and still only spend $13m for all five of them. If a position is inexpensive in regards to the total cap, why not get the best out there. It isn't like we had to spend $13m for one of them, its $13m for all of them.