Realism is always tricky, because there is only one sample (Real Life) to use as example or model and by nature it is basically impossible for a fantasy world which (by definition) is already not realistic and thus it might not have to develop in a necessarily "realistic" way.

That aside, there is probably are no comparison between the different Human species who were very similar with each other (with there also being evidence of interbreeding with traces of non-Homo Sapiens Sapiens (regular humans) DNA been found in modern humans) and different sapient non-human species which are not able to interbreed with each other.

I don't know if you understood what I wrote (I don't think I worded it well), but what I'm trying to say is that the similarities of the different Homo Sapiens species could have very well be what contributed to only one of them surviving to this day and age (possibly due to the species intermingling).