Yeah I really don't see how one could describe Andersson as having 3/4C upside. That, to me, is his floor. He's a really, really safe bet to be no worse than a 3C. Unless you think he has no room for growth, which is preposterous, then there's definitely more upside to him than that.
I think that Andersson is pretty close to an NHL 3C right now. Honestly, had he made the team, I think he would've progressed as the year went along and had a similar year to Bergeron's rookie season (35ish points). How many players are at their ceiling at 18? And that's not to say that him going back to Sweden is a bad thing. Bergeron played 13 minutes at even strength per game that year, which was good for 2C on that Boston team until they acquired Nylander. You'd expect the Rangers 3C to see similar time.
The biggest difference here is that for most of that season, if Bergeron wasn't on the team, the next best center the Bruins had was Travis Green. With the Rangers, you already have Zibanejad and Hayes and while Zibanejad is no Joe Thornton, Hayes is a damn sight better than Green. Plus JT Miller, who will get time at C if Chytil ends up with nothing more than the 9 games. In other words, the Rangers don't
need Andersson on the team, even without Chytil's performance. If the Rangers were flat-out rebuilding, Andersson would've stuck around.
In the end it came down to Hartford or Sweden. While there was a ton of hockey benefit to be had putting him in Hartford, I think the personal benefit of playing in Sweden ended up being the deciding factor. This guy is likely future captain material. Let him experience the personal growth. The hockey will come along with it.