The idea of Yamato joining the crew sounds horrible to me to begin with, but some people's attempt to put him in the same group as Zoro (first person to ever join the crew), Sanji (only crewmate to have two detailed flashbacks and a whole arc revolving around him apart from his introduction) and Jinbe (took literally years for him to finally join, we've known him since pretimeskip and was vital for Luffy to cope with Ace's death) just hurts me. If there's someone who deserves to be Luffy's fourth commander is Usopp, plain and simple.