Is diplomacy necessary?

Having many powerful creatures in one army without the presence of the Haven faction is a direct path to defeat. The moral of the story is that good doesn't always win.

Well, I don't know... of course, if you play for 5-6 months, then yes, there will be enough different creatures, etc.
When playing online, a small number of creatures (even just one type) is enough to put up some resistance... and morale of 3 can happen, but don't forget about leadership and various high-morale items, such as an amulet (+3 morale + 3 luck). So, you just need to play skillfully with diplomacy.