At the time of the trade, there was immense value for the Sharks in having Karlsson for a year - where they could try and get him comfortable in San Jose and extended to a contract.
There was talk at the deadline that Vegas KNEW they were one of Stone's preferred destinations, if not the top preferred destination. They did not gamble and wait to acquire him on July 1st because there was a fear that if he went else where, he could get comfortable and eventually re-sign.
Even if Karlsson walks, it does not invalidate the value in the above.
Ignoring it would be like ignoring that a 1st round pick had value at one time just because the player taken with that same pick has busted.
They gave up a 1st, a 2nd, two B prospects, a middle 6 center with 3 more years of team control, and a fringe NHL defender for 1 year of the best defender of this generation, and for being the only team able to offer him 8 years (and have an entire year to woo him into re-signing). There is also a conditional pick if he re-signs, but most people are approaching a critique of this trade from the standpoint that they don't think Karl will re-sign.
The return becomes even more ridiculous when it is recognized that he 2nd they gave up was produced via hustling Ottawa in the Boedker/Hoffman debacle. The Sharks moved a player that likely was unmoveable else where for a 2nd round pick via flipping Hoffman.