Cavemen are considered primitive and even if they were just blue/green rats one day and instantly trolls the next day, it doesn't mean there wasn't life or species before it.
The fact that Blizzard calls it evolution and not rat instantly transforming into trolls should make you realize there is stuff inbetween x and y.
And you hand waving elementals and shit because you personally don't believe them to be a race doesn't magically make then not one.
Also notice how trolls vary by tribe? If they didnt evolve then they all should look like Proto Zandalari. Just because they keep the name troll it doesn't make them more special then the elves. It's even more funny that without the Titans intervention trolls would have never existed.