Sadras
Legend
It's not that the warlord couldn't be a sub-class. It just can't be a sub-class of the current 5e fighter.
Is your basis for this statement purely on the reason that Warlord would supposedly have to give up multi-attack for inspiring word? Your argument then appears very weak. Just swap out the normal Battle Master maneuvers for warlord-y-like abilities why is this such a huge deal still?
Furthermore one needs to take into context the style of game 4e is with what 5e actually is. Granted 5e is supposed to be inclusionary, but it certainly doesn't have to be an exact replica of 4e. If you want a 4e warlord so badly, I suggest you play 4e.