It's really hard to understand, it wrecked my brain trying to figure it out too, but I think part of the problem is that we don't know exactly how the Titans create/destroy worlds, from what we have seen and read, Algalon was going to destroy azeroth using some sort of planetary orbital weapon system, luckily we were able to stop him from even thinking about it.

Yet creating systems seems to take them millions of years and they require the aid of the local inhabitants to do it.

Your right, its weird that us players are able to achieve so much, and it makes sense that newer and more powerful enemies keep popping up, but I think we are going to have to fight a Titan one day (I'm guessing Sargeras) and I look forward to it and the answers it brings!

YOu have to remember, the titans didn't want to kill the old gods, just contain them.

If the old gods were killed, all of the life that the curse of life touched would die....

So, ready for all of our characters to die?

Remember this from the Tribunal of Ages in the Halls of Stone:

Abedneum yells: Accessing. In the early stages of its development cycle Azeroth suffered infection by parasitic, necrophotic symbiotes.
Brann Bronzebeard yells: Necro-what? Speak bloody common will ya?
Abedneum yells: Designation: Old Gods. Old Gods rendered all systems, including Earthen defenseless in order to facilitate assimilation. This matrix destabilization has been termed the Curse of Flesh. Effects of destabilization increased over time.

So that's where they came from. And it's pretty much established that Yogg-Saron and C'thun aren't totally dead right now, so we haven't just killed them, just knocked them unconcious for a bit.

Algalon wasn't going to destroy Azeroth; he was going to 're-originate' it. The process would've killed everything currently on the planet (presumably including the old gods, since Yogg-Saron's imminent escape was the trigger). But the Titans didn't want to destroy everything on the planet; tainted or no, they perceived value in its inhabitants.

It's also terribly likely that neither C'thun nor Yogg-Saron are entirely dead.

The trigger was the death of Loken.