A few nitpicks:
I don't think one person thinks he won't be better than Ekblad, but I do think it's possible his rookie season isn't as good as Ekblad's, for three reasons. One is that he is guaranteed to not have a partner as good as Campbell was that year. The second is that Phil has shown no inclination that Dahlin will get any sort of optimal usage, because Phil's usage is whacked out in general. Ekblad's was literally perfect. And third was, I don't think for a second that Dahlin is weak or not strong enough for the league, but Ekblad was already a 28 year old man during his rookie year, and was stronger than most of his opponents like Jack, which helped a lot. It's possible that Dahlin takes a little longer, is all, just like Hedman, Doughty, Karlsson all had under 22 points their rookie years.
Of course, it's also possible he is Orr incarnate, and it's fun to dream while it's still summer. But I think Sabres fans on balance have a pretty good distribution of rookie year expectations for the kid.
Also, we shouldn't be throwing around Karlsson offense and Lidstrom defense lightly. The combined package of those two things is, without exaggeration, the 2nd best defenseman in NHL history. This is guaranteed to no prospect ever, since it requires light-speed development and absolutely maxed potential, neither of which can be even close to guaranteed.