Well, we can't send Atkinson down even if we wanted to – he's on IR. Our other "young players" include our defensemen, which as we all know are in short supply right now, and Matt Calvert. Why not send Calvert down? To me, it's because he is playing the role we want him to play, and the barometer of success/confidence in that role isn't quite as tangible as Johansen's. I think Matty merely being on the ice and being trusted can do a lot for his development given the role that we want him to play. His scoring chances are going to come the same way his whole career: by using his speed, work ethic, and opportunistic instincts. He's in a position where NHL experience is a better chance to hone his craft than top minutes in Springfield.
This isn't to say Johansen wasn't filling his role adequately (IMO he was playing better hockey than pretty much anyone else), but it's kind of a waste to see a kid like #19 do all the right things and not get rewarded for them because he's playing with guys who aren't on his level. Never in his career is he going to be the straw that stirs the drink, but what he will be (if he's developed properly and surrounded with guys that can keep up with him) is a guy who you know will put himself and the puck in the position necessary for good linemates to capitalize. A player who you won't notice make a play until the puck is in the back of the net.
My point is, the arrival of those linemates may be just around the corner (Mackinnon, Drouin, Barkov, Shinkaruk, Lindholm are guys that we could be in position to nab, and they would all be great fits with Joey). I'd rather him be an important piece and gain experience than just be put to waste in the NHL, even if he does belong there.