I don't buy it, well not completely anyway.
If Karlsson walks and the Sharks don't win the cup they essentially traded everything they did for Francis Perron -- and that would be highway robbery -- in Ottawa's favor.
Without a guarantee that Karlsson will resign with them, a team needs to be either a legit cup contender hoping he will push them over the top (and don't care if he resigns or not) -- or confident enough that they'll make a good enough impression on him that he'll want to resign with them.
That eliminates any team that isn't likely to make the playoffs (Arizona, Vancouver, Chicago);
It also likely eliminates the bubble teams (LA, Calgary, Edmonton, Anaheim, Dallas, Colorado, Minnesota);
Of the 5 teams remaining in the west Nashville needs a defensemen less than San Jose did;
St. Louis didn't have the cap space;
Winnipeg would have worked, but it's Winnipeg, so the chances of him resigning are lower (sorry);
Vegas might of worked, but Vegas might also crash and burn this year with players coming off career years -- so too many unknowns and would probably make anyone other than Ottawa skittish about a trade;
San Jose is of course where he was traded to.
Really only the Sharks were available in the west that could be confident enough in their performance this year that they'll stand a decent chance of resigning him, and liking their cup chances enough to take a shot and not care too much if he doesn't.