Personally, I disagree. Sacrificing Malfurion, while honestly a pretty awful way for him to go out, serves to set up so many potential pieces for WoW into the future. How does Tyrande lead her people out of grief knowing that her beloved Shan'do is gone to the Shadowlands & not coming back? What becomes of the Druids with the main Archdruid gone never to return? For that matter, if we do wind up going back to Ardenweald for whatever reason, what sort of effect has Malfurion had on the eternal forest, & what effect has it had on him? He's powerful enough at this point that it isn't out of the question in time he could become a new Wild God, similar to Vol'jin seemingly becoming a Loa.
But it's not just that. Sacrificing Merithra for Ysera also has the issue where we would be losing the future leader of the Green Dragonflight for the past leader. It would be somewhat akin to Kalecgos being sacrificed to bring back Malygos, though without the ending part of Malygos' war on mortal magic attached. If Ysera is to come back, she'll likely be there in part to help give a bit more of her time & aura to Merithra & build that character up for the future. It also fits better in the story for Malfurion to be there instead of Merithra as there is a story reason why Malfurion could have fallen...Merithra doesn't have that same background at this point.