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As long as enough fans want a warlord that WotC will profit from giving or selling a warlord class to those fans, they should do it. That doesn't have to mean a large percentage of fans. It could potentially be as low as five or ten percent.
It's actually not that simple. Consideration goes beyond "Will WotC profit from selling a warlord?" It has to include "Will they profit more by doing something else?"
Let's say a full-fledged class requires 6,000 words of work. (I'm not saying that number's accurate; I'm just using it for illustration purposes.) 6K of words has a fixed cost: X for the writing, Y for the editing, Z for the accompanying art, etc.
The question is not, "If we spend X, Y, and Z on a warlord, will the book sell?" The question is, "If we spend X, Y, and Z on a warlord, will the book sell better or worse than if we spend them on [psionics/wardens/new feats/whatever]?"
And that's just the measurable costs. Things like "What fits best into 5E as we see it?" or "What fits best into our plans for this year?" must also be taken into consideration.
I'm not saying the warlord will necessarily come out second-best in such considerations. I don't know. I don't have access to WotC's research, nor do I have a dog in the warlord race. I'm just saying, it's not nearly as simple as "Does some of the audience want a warlord?"