"People are starting to assume that if a character doesn't have haki they will be just fodder."
that's an oversimplification, i would ignore those
"Also noted that haki reflects your will. We have been told that the vinsmoke siblings lack emotions so it is possible they lack motivation on their attacks."
this is an interesting take
sure the part about them lacking emotions sounds still blurry, but it would be interesting if their bodies are so strong they never had the need to learn haki cause their raw fighting ability was already high enough to beat any enemy they ever faced, even those using armament haki
yet, we saw how Yonji's head was modified by a kick from Sanji...
so they may not have learned haki not because they were able to fight with raw power(+ their abilities) enemies with haki
but because they did most of the time clean up jobs, which involved killing armies of a nation which may not had many soldiers with haki in them, while still giving them the impression to be invincible
or they could rely more on tactics (like using their clones as meat shields) over direct raw power from the members of the Vinsmoke family