The core isn't built. Montreal's got one line and a couple of D-men, and we can't project their upside (or the fit of guys like Demidov and Reinbacher). Montreal may have a core. but its not there yet. And you have to verge on the level of homerism to project the contention window as being basically next season if the idea is to add a 38 year old to add to it. Even if that player is Crosby.
I don't have any issue with Montreal moving picks/prospects/etc. to add to the core. I have a problem when its adding one of the oldest players in the league.
I generally don't buy the argument that 3 picks gets Crosby though because there's no evidence he wants to play anywhere other than Pittsburgh and there's plenty of evidence (from the fact he signed a contract last offseason to his public comments) to support it. And if we're daydreaming, I'd rather daydream about McDavid not believing in Edmonton's front office and wanting to lead a pre-eminent franchise to a cup.
I also don't think Crosby makes Montreal a contender right away, because Montreal's bigger issue isn't offense, its defence. And Crosby isn't the two way force he was even a couple of seasons ago. And if Montreal doesn't address that then they can be a top 10 or top 5 offensive team in the NHL and not contend (and may not even make the playoffs).