Quote Originally Posted by Vahnavoi View Post
If, as you propose, sapient creatures speciate based on dominating their ecological niche, why would they see each other as competitors? To give contrast from natural world, ants war against other ants (and termites) - but only those ants which happen to be of similar size and eating the same food. Small ants and big ants co-exist comfortably because they are not each other's direct competitors.

Approached from a different angle, the advantage required to carry out complete, intentional genocide is vast. The simplest explanation tends to be that neither side is capable of exterminating the other. The second simplest is that the other side is trying hard, but you're working to stop them - that's what the fight against forces of evil genuinely is in many settings.
There could be some tension between men and elves. Elves tend to be fond of the forest while men tend to be fond of cutting forests down. I don't think anyone would bother the dwarves because nobody wants to live in a mine shaft their whole life, so dwarves are useful to the economy because they mine ores nobody else would bother with. Most other creatures live in remote and hostile locations which is probably why they survived this long.