I'm not sure about the purpose of the Halo rings. From what I understand, they kill sentient life to starve the Flood while leaving people in shield worlds to wait it out and restart civilizations. But if they don't fire the rings, what could possibly happen that would be worse than what the rings actually do?
There's a chance of biological balance, the predator-prey populations regulating each other. There's a chance that they find a way to kill the flood and so civilizations continues as normal. According to Halo wiki, it's been done before. Otherwise the flood kills everyone and starves anyways.
It seems kind of like a roundabout solution. Wouldn't it make more sense just to send people to shield worlds and wait for one of the three previous scenarios rather than killing the entire galaxy?
The Forerunners probably foresaw the Flood Forms becoming the dominant species in the Universe and then raising other sentient races as cattle.
As another example, consider the movie, The Time Machine with the human race divided between the subterranean morlocks and the surface dwelling eloi. The morlocks used technology and the eloi were cattle; raised in a simplistic existence to be taken below and eaten by the morlocks.
The Halo Array can only be understood as a strategic element of a larger plan, including installations (like the Arks) that are outside the galaxy, and Shield installations that are immunized from the Array.
What strikes me as peculiar is the purpose of keeping the Arks so (comparatively) close to the Milky Way when the original 12-ring Array would have fired MUCH farther than the 7-ring Array. Do you think the Ark used to be farther away?
In the newer books and the anime they sort of show that while species are preserved, they aren't preserved in their real form, simply their DNA is saved and then restored once the Flood was presumably gone from the galaxy.
The problem with "waiting" it out, is space limitations. You couldn't store every Forerunner inside the shield worlds and if they had let the Flood go, what Dust said is presumably what would happen. Then they'd grow to a stage where they wouldn't need food anymore and then spread to another galaxy.
The Flood were momentarily halted by humans and I want to say the San 'Shyuum (Prophets) long before the firing, but they were defeated by the Forerunners.
Also, the Forerunners really had the "ace up their sleeves" which was cloning technology. They wiped the slate clean and when it was determined to be "safe" enough for sentient life, they had the species cloned back into existence. It was the last fail-safe plan, but needed to be done as by the time they were fighting the Flood, it had grown too large in power to stop by conventional warfare. Even destroying entire solar systems did nothing to stem the tide.
So yeah, they stopped the Flood by starving it, the only "sure" way of preserving life in the galaxy.
Have you read Primordium? It contradicts two things you said: 1) The Arks (or at least the lesser Ark, but probably both) were genuine habitats for living individuals. The Didact tells Chakas (as he's being converted into 343GS) that Riser and Vinnevra have been given a habitat to live in on the Ark. 2) Ancient humans didn't cure the Flood; the Primordial said that the Flood had the ability to choose whether or not to infect a subject, and that most humans were deliberately not being infected as a part of the Precursor's overall plan to overthrow the Forerunners and "test" the ability of humanity to assume the Mantle.
Actually, they do not contradict themselves. The Forerunner saga has been the one with the less contradictions so far. The Arks being habitable is rather logical seeing how they are outside of the Halo Array's range and they are massive enough to house different ecosystems on their own.
The Array itself is a way to get rid of any sentient life in the Galaxy (The pulse also affects the Flood if we go by Halo: Legends and Halo: Combat Evolved Anniversary's depictions of the firing.) And well, the Forerunners had over 400 shield worlds, so they had more than enough space to repopulate (the Dyson Sphere inside Onyx was measured at 2 AU [29,919,5741.4 kilometers]) which is exactly two times the distance between the Sun and Earth. Now, the new shield world (known as Requiem) that we are visiting in Halo 4 might actually have Forerunners in it.
And no, the Flood would not grow species like cattle, once they absorbed every single species in the galaxy, they would leave it and infect other galaxies, their goal is unity through infection (as there would be no more war nor pain or suffering, it's basically paradise in exchange for your own will), a design the Precursors probably based on their own idea of keeping the mantle (seeing how Forerunners bastardized the concept). But anyways, the last bit about Precursors is only conjecture from my part, I haven't read Primordium in a while so I'm kinda fuzzy on the details right now.