Loken was corrupted. We killed him. That triggered Algalon. Was Loken the only trigger?
It seems rather short-sighted to have only a single warning system. How was he intended to work anyway? The plans of the Titans are hard for mortal minds to decipher.
Can Loken trigger Algalon himself? If so, then this would allow Loken to act as a sentry. I imagine this was the original intent. Loken dying was the backup, that if he died something was seriously wrong.
While us killing him did activate Algalon, it seems unlikely that the Titans set up the system as Corrupted Loken -> Dead Loken -> Algalon. A corrupted world should have been detected and reported by Loken, before he was corrupted. That he was one of the first to be corrupted ruins everything. Would the Titans have expected the world to kill Loken and thereby activate Algalon? I doubt it. He wasn't supposed to be a pushover. Besides, if Loken was corrupt, isn't it likely that the world would be as well?
It is a strange irony that it is only our perpetual greed and love for violence that allowed the Titans to have any idea that Yogg-Saron was released and corrupting the world. By that I mean, if we weren't so bloodthirsty, we might have not killed Loken but instead imprisoned him. That would have effectively disabled the trigger since I doubt Loken was planning on drawing the Titans here to remake the world, again.
All in all this looks to have been a poorly designed system. The corruption of a single entity was sufficient to disable the warnings about Yogg-Saron. If we had not wandered in to kill Loken and take his stuff, the world might be lost.
I will accept that things have turned out okay in the end. Algalon did see that we are not so awful after all. But this appears to have been a special case. From the sound of it, any other world in our situation would have been burned down. However any other world in our situation, but without our heroes, would have both been unworthy to Algalon and also would have not triggered him. In other words, if not for the chance emergence of a small group of heroes, Azeroth would be ruled once again by the Old Gods and the Titans wouldn't even know unless they were already on their way for a routine visit. The early warning system failed.
Was there a backup system? I suppose there are the routine revisits, but how soon will that be? Were the Earthen the backup? They were admirable defenders and never gave up, but let's face it, they were losing to Loke's Iron Dwarves until we came along. Maybe the races in general were believed to be the backup, but they were just as easily corrupted.
The Vrykul thought the Titans had betrayed them and ended up turning to the Scourge. Even if they had remained loyal they might have fared no better against the Iron races than the Earthen. It was their corrupt form, humans, which successfully fought back.
Were the trolls expected to help? I don't know if they existed when the Titans were around. If their emergence had been anticipated, then they weren't a long-shot. They did subdue the initial Silithid (Aqir) though I'm not sure they were part of the Old God's schemes yet so that might not count. Besides, the trolls summoned Hakkar who is of comparable evil as the Old Gods. They became the elves and it was the Highborne who nearly brought Sargeras and who are now allied with an Old God in the Maelstrom. Currently the elves and trolls under the major factions were useful in fighting off the Old Gods, but that was hardly a guaranteed security.
There were backup defenses, but for actually warning the Titans, it appears that Loken was the weakest link in a single chain and without him it all falls apart. His susceptibility to corruption was underestimated, or Yogg-Saron was underestimated. A lot seems to have been gambled on the ability of the races to defend Azeroth and on the effectiveness of Loken as part of the early warning system. This isn't an isolated incident either. C'thun's defeat at the hands of the Old Gods was temporary at best and he was not contained adequately. It was only the intervention of the elves and dragons that saved the world.
The dragons were another defense system. How reliable are they? Ysera is falling to the nightmare. Malygos turned on the world. Deathwing was absolutely corrupted by the Old Gods. For the Titans to have expected the aspects to keep effective watch was clearly asking too much. While the dragons did play the large part in defeating Deathwing, the actions of mortals were not inconsequential.
I hope you're noticing the theme: The Titans failed to adequately secure this world against the corruption of the Old Gods and now the mortal races are stuck fixing the problems, struggling to stay alive. We've also been busy fighting with the Burning Legion with mixed success; the fact that we've survived puts us at the top of the rankings for how worlds are faring against the Legion. The Titans would have very likely wiped us all out because of their failure. Is that not absurd? We defended our world far better than they did, we preserved some amount of order and good, and we'd have been consumed by the very same fire as the Old Gods.