Huh? Giles and Bagley are supposed to be the future frontline. Those are the upgrades, unless you want to keep one of them on the bench. Then you are looking at an all-star level free agent, unless your point is that both guys are PFs.
Neither of them have the shot to space the floor consistently like a true "stretch" big. Giles still is hesitant to fire 17 footers. Bagley too, as he looks to attack the basket unless he's already standing outside the line and is wide open. Being able to hit a shot is a much different thing than being an offensive threat from that range. Nemanja is the second best deep shooter on the team behind Buddy, who is one of the league's best.
Now, if the idea is playing them both at the 4, and picking up a very good center to go along with them, then that's not a bad idea. Bagley can split minutes between the 4 and 5 as well, but he will have a tough time against the bigger and better centers of the game. If we got a guy like
Valanciunas I think it would allow Bagley to out-play the forward they put on him, but it won't stretch the floor. However, with two players of that calibre, they will feast on single defenders. What will end up happening is Bagley will start to get doubled, and when that happens, you don't need a stretch big to space the floor, because the defense is already askew with the double. Ball movement will result in an open shot or ability to break it down and get to the rim.
Nemanja doesn't need to be replaced, he just needs to come off the bench if the team upgrades to a better center. I would much rather they go after a quality center than waste money chasing a guy like Mirotic or Tobias Harris which would force Giles and Bagley to play out of position.