The question of how long the window to winning a Cup will stay open for the Habs is an interesting one, even to a Leafs fan. My first answer is that the Habs will stay a playoff team and at least have a shot at the Cup as long as Price remains the best goalie in the world. Goalies are inherently unpredictable so who knows how long that will be! It could be a number of years, however. However, even a superstar goalie is not enough to make a team a Cup favourite by himself. Cup winners normally have not only at least a very good goalie, but also a top defenseman, center and winger. I suppose the candidates for those roles would be Price, Galchenyuk, Pacioretty and Weber in the case of the Habs. The one I would be crossing my fingers on, were I a Habs fan, is Galchenyuk. The one whose best by date is approaching the fastest is Weber. I think the answer to the "how long" question is perhaps "as long as Weber remains a star blueliner."
Teams also need depth of good players, however. Acquiring Phil Kessel certainly helped the Pens last season in their Cup run. I think acquiring JVR could have the same effect, though there are obviously no guarantees. It may also be that acquiring a winger is easier than getting a very good DMan or center. But... if, if, the Habs are so eager to make a Cup run this year that they are willing to give up their best prospect by far, would spending the asset on a winger actually be the best move?
Just by way of speculation, what if the Habs had a go at obtaining Tanev? I know there are interminable threads here on the general subject of a Tanev/JVr trade. Maybe there have been threads just as long on Tanev to Montreal. But I am just using the idea as an illustration. It isn't a proposal so if there are Canucks fans lurking here, Peace! I do think that trading Sergachev for a really solid Dman would be a better use for Montreal's top asset than trading him for a winger. If I were the Habs GM, I think that would be the direction I would be exploring first. Get a good enough Dman and maybe the best by date on Weber doesn't matter as much, so it could also be an investment for the future.
But who knows, maybe the Habs will come back to acquiring JVR later, even if he doesn't fill their most pressing need.