I think comparison is a bit unfair since nms is (mostly) single player while fo76 is not. If nms gets good (i believe it is good btw, I play it regularly on VR) it would still sell 5 years later but fo76 would be dead if it can't keep up player count high.
So yeah, a failed multiplayer/mmo game is a bigger failure imo, because it is hard/impossible to recover from
