It's the only way to be sure.
Once it does so, this lone Reaper (either a fully intelligent entity of its own accord, or a new body for the original Reaper to transfer its programming into) can leave and repeat the Collector's method of gathering bodies. By taking only most of the people in a colony, and indoctrinating the rest, it can then get servants inserted into galactic civilization, which would work to find positions allowing them to cover up the Reaper's actions. Over time, the Reaper would be able to create at least one more Reaper, which would allow more daring action. By picking off colonies and retreating when confronted with large forces, they would eventually be able to create a small force of Reapers. Utilizing their indoctrinated servant (and the following they would amass), the could instigate a galactic war, using the chaos to pick off larger planets for faster collection. By carefully managing and extending the war as long as possible, they could wear down at the military forces of each side (attacking production sites and blaming another faction would accelerate this). At the lowest point of the war, they act to take out the remaining military forces, and as each Reaper is easily worth a goodly number of other vessels, this would be a battle the Reapers could win (especially since they'd use tactics other than absolute, overwhelming power like the Reapers normally do when they fight). With the military of the galaxy largely destroyed, most of the force would rest in groups like pirates and other criminals with money. They are most definitely not going to work together well, allowing the Reapers to go harvest highly populated planets with little fear of repercussion (although they would still have to do brief, routine checks to ensure the rest of the galaxy isn't getting its shit together and making more weapons). Take the stealthy approach for another three or four cycles, and they'd have enough numbers that the chance of even a ready-for-war galaxy would have little chance of stopping them.
Of course, any of the three not-useless-suicide endings would invalidate this... but from their knowledge of what the Crucible was meant to do, they had no way to know that the Reaper would be affected inside a planet.
After her titanic battle with a Reaper Destroyer on Tuchanka,Kalros went into hibernation once again. Unfortunately,the Reapers took notice of this event and managed to capture and convert Kalros into a gigantic modified husk of her old self,the Typhon,a namesake fitting due to her stature as the largest organic to have ever existed to date. With these new upgrades,the Typhon was outfitted with a cannon similar to a Destroyer's that was placed in the mouth region,as well as several smaller mass-accelerator emplacements running down the length of her body,all complimented by heavy armor plating across its body. Despite these enhancements,the Typhon retains the main style of attack as other Thresher Maws,rising up from below and utterly crushing any opposition. Allied forces are advise to steer clear of this massive monstrosity,and are ordered not to engage under any circumstances.
Dudes, if the thresher maws became reapers, Tuchuncka would be SCREWED!
Awsome design. It really looks like its been "harvested and repurposed" to serve the reapers.
Great pic though
Then again, that thing might have machine gun jubblies...
In a way, this would have been a more plausible outcome.