[grondilu]

That conception of what a species is has been my understanding as well for a long time. It's a very simple and clear concept but apparently biologists have given up on it for various reasons. There's actually a whole wikipedia article about it:

https://en.wikipedia.org/wiki/Species_problem

[mikeash]

That's because the ability to mate and have viable offspring is not anything like a simple and clear concept in some cases. For example: https://en.m.wikipedia.org/wiki/Ring_species

[Steko]

Nothing in your link suggests they've all given up on the concept.

[grondilu]

Not the concept itself, but the aforementioned simple and clear definition.