What is wiping out the protoss

[Alex06]

It considers humans a weak source of food due to their low level of psionics. Morrigan was psychic, so she would have lasted longer. There's a reason it preferred protoss to her, though; she was only a weak psychic.

But perhaps it managed to take control of Duran somehow differently (He was stronger, better, more evolved?), or it placed him there as a puppet, or even, perhaps Duran is not actually a human, but something that has taken human form. Perhaps it is an extension of the voice...I'm sure there's a way that Duran could be the voice's slave without having expired do to his human form...

And, Norfindel...
http://starcraft.wikia.com/wiki/Morrigan

[Gradius]

It can't have controlled Duran. The Voice was locked away the whole time. It could be a similar entity though, or just something with similar powers.

[GnaReffotsirk]

The voice was energized by the Psionic flux created when Tassadar channeled Zeratul's energies against the Overmind. The Crystal shards that was made to keep the Voice limited got into contact with the Khalai Executor, and ergo the plot of SC:Survivors.

Sorry for the plug.

Seriously, I think that since the Protoss range upto 1k years in life-span, considering the events taking place, it would seem like the Protoss are an endangered species. For all we know, they reproduce selectively, as in only a few are "ordained to give birth" (either by edicts of the conclave, or by genetics, either naturally or due to the Xel'Naga's experiments). Considering their very war-like society, their strict adherence to their codes reflected in the conclave's stern and sterile rule, their apparent lack of interest in mating and establishing some social circle, having much less privacy, etc. we can safely say that the Protoss do not take loses as would Terrans or more so the Zerg.

They've been "dying" could be related to their way of life in terms of their principles, the conclave's rule, and their "I am, and one with all" attitudes. I'm certain the influence of external powers are less noticeable. If not, they would have felt it and done something about it. This subtlety could not have been known to Dustin at that time. This element (if it does exist) is too deep and recessed to be the immediate image he'd conjure and cause him to say that line in response.

[Roland]

This has gone about as off topic as possible... It's not about the Voice, it's about what's been happening to the Protoss. For the record though, the Voice provides an interesting theory as to what Duran referred to as a "Far greater... Sleeping power."

On-topic though, The Protoss probably haven't stopped evolving since the Xel'naga left, and they've probably kept on the same paths that they were put on before the Aeon of Strife. We also know that one of the fundamental traits possessed by every generation of Xel'naga, is the inability to reproduce. I also assumed that was because they're all hybrids, which usually can't reproduce (see mules). But it's possible it's a side effect of one of their parent races. Maybe the Protoss, as they've grown into having a more perfect "form" have been losing the ability to reproduce somehow, which albeit doesn't make much sense from an evolutionary standpoint but assuming the Xel'naga were only evolving them for Purity of Form, when they left, the Protoss evolution may have just stayed on that track of maximum efficiency at the cost of other traits.

So as the Protoss evolve, inevitably more and more of them grow to lack the ability to reproduce (which cannot be passed on to future generations for obvious reasons, but imagine that every Protoss family tree is heading towards the same spot, once one family ends because of the inability to reproduce, another family will eventually develop the same trait, and end, this process would continue until extinction.) So as fewer and fewer Protoss can reproduce, there death to birth ratio continues to get worse, meaning, they're going extinct.

It's also possible that the Dark Templar are the future, as they've clearly evolved separately from the Khalai, and away from the Xel'naga vision, they may still be able to reproduce.

[Visions of Khas]

So as fewer and fewer Protoss can reproduce, there death to birth ratio continues to get worse, meaning, they're going extinct.

For a race to lose the ability to reproduce seems... off to me. Especially considering the fact that Artanis is recognized as being fairly young. However, if you're theory is correct, then it would seem the Protoss are designed every which way to merge with the Zerg, or face extinction, whether or not they pulled themselves from war.

[Triceron]

One of the problems is finding a source of mature Protoss to replenish the ones lost in the first war. Motherships are certainly one source, and the newly found Dark Templar tribes are another.

The problem is after these 4 years and the second coming of the Zerg and now the potential threat of Xel'Naga/hybrids, how would the Protoss survive? They don't have any more wildcards, this is it for their entire race. The time it takes for them to mature is likely hundreds of years; how would they be able to replenish their ranks?

[Roland]

It looks like Legacy of the Void may turn into last stand of the Protoss. If the old Protoss government didn't want to reunify with the Dark Templar, just think of their reaction to the Zerg.

[Visions of Khas]

how would they be able to replenish their ranks?

The Khassia crystal was a bauble in one of the manga that had the ability to either speed up or slow down reproductive rate of Protoss and Zerg. I wonder if something like this would make an appearance in the game? Seems a bit far-fetched a concept, though...