I don't think they can ever totally stem the loss of subs at this stage in the game's life, but if they're ever going to slow it down Legion seems like their best chance. Two months in and I still don't have one legit complaint, and the majority of the complaints I've read from others (flying, not getting a legendary ASAP, RNG, Underbelly PVP, whatever) is really all just petty bullshit.
Legion's off to a very solid start. If it doesn't stop people leaving, nothing will.