They know who Necas is - a passenger, complementary kind of player. This breakout was nice but likely not the reality of who he is or will be, based on their past experience. More importantly, they have the cap room to give Rantanen something close to what he wants, plus they will be turning over a lot of the roster to ELCs into a rising cap. But say, Rantanen leaves - you got, by far, the best deadline acquisition for a team trying to win now in exchange for a guy who was likely leaving in a year anyway based on contentious negotiations.
I'd also put money on an aging Rantanen still being a better player than peak Necas.