Steelers should sign Johnson
Just like edge rushers, the running back position is one dominated by the starter but lacking depth elsewhere. If Najee Harris were to go down, this offense would be in a bad place. Even if he stays healthy, this team could use a veteran to help keep him fresh. The options currently on the roster are lackluster, and the Steelers would be wise to seek a veteran out.
At one point in time, David Johnson looked like he was going to be the next big thing at running back. He took the league by storm, but injuries slowed him down and a tough stint with the Texans leaves him as a free agent this late into the offseason. While he lacks the impact he used to have as a starter, Johnson is still a tough runner and can fill in if needed for an offense.
While his best days are behind him, adding him to the roster could give the team a DeAngelo Williams-type of back to compliment Harris. As well, if he were to miss any time, Johnson could start and give the team a similar style of complete power back (he was a decent receiver out of the backfield). This offense clearly wants to run this season, so having a capable backup at the position makes sense.