The problem for Puljujarvi now is that the Oilers are actually good. If he comes into training camp, he probably gets a look in the top 6, and if he had succeeded they would have kept him there. It's not like he didn't get to play in the top 6 at all while he was here, I don't see why that would've changed.
I think what he wanted was to be given the opportunity to work on his game while playing in the top 6, which like it or not is not a strategy a lot of teams utilize. Some do, but in most cases if a coach feels a player is struggling, they're going to move him down to the bottom of the lineup until they start making things happen. Players like Josh Archibald were able to do that, and then when the opportunity comes, they get a shot in the top six. That's what Puljujarvi had to do, and probably will still have to do if he comes back.
I just don't buy that being a third liner for a couple of years is that detrimental to his career. His camp obviously feels like the Oilers were burying him and therefore hurting his ability for future earnings, but I have to say I just don't buy it. The Oilers want nothing more than for Puljujarvi to be every bit the superstar he thinks he's going to be. The only criticism I have for the coaches is that I think he probably should've gotten more PP time when the PP was struggling, but that's gone now (I don't think he's going to come back and play a net front role).
And I also have to say, I just don't think it's that bad to be a third liner in the NHL, nor do I think receiving criticism from the fans in Edmonton would affect these guys that much. They're making millions of dollars, and generally even the guys who fans turn on are treated like gods in this city. I think it's more likely that he experienced some culture shock and didn't fit in with the high end players in the room, and felt like that's where he belonged. I think there's zero chance he left because the fans were too hard on him.