But shouldn't we look back on trades time has past, and not the day after?
Well, a trade is made on a certain date relative to the value of the players on a certain date. For example when you trade a player for a draft pick, you trade for a draft pick (probability of getting a good NHL player down the road) and not the drafted player because you don't know who'll get drafted with that pick. Then there are many other variants that come into play like scouting staff and development.
You can't really evaluate a trade in retrospect only.
For example what if Taylor Hall suddenly becomes a bad player next season? Will the Oilers have won the deal in retrospect? No, not at all because the value of Hall was much higher than Larsson when the deal was made. This is an unrealistic example but you get the point.
If you make a trade for draft picks and those draft picks turn out great then it seems like you've won the trade, if those draft picks turn out bad then you lost the trade but that actually has nothing to do with the trade ... It has to do with all sorts of other things. The value is in the draft picks, what you do with them and what they turn out to be after depends on many variables not related to the trade itself.