I'm not understanding the displeasure of Fast on the top-line. By himself he isn't a 1st line player but whoever plays with those guys isn't going to have the puck much. It's go get the puck and find either Zibanejad or Panarin and give it to them. He's intelligent enough to find soft areas and make good plays with the puck.
When the Pens tried Hagelin, Dupuis, Kunitz and many others with Crosby and Malkin over the years did people see them as 1st liners? of course not. They had their role on the line and were able to get pucks to their more skilled line mates who could make plays.