The 49 sentinels could destroy a Covenant cruiser because they were explicitly designed to form up into highly configurable patterns that made use of their exponentially increasing power for the more that formed up . That is not standard Forerunner tech . First of all , nothing anywhere mentioned exponential growth in power . Second of all , logic does not support this either . Why would the Forerunner even build larger craft ? Just pile up more and more sentinels if they power up exponentially . If 49 can take out a covenant capital cruiser , then imagine what 49 x 49 can do . That ' d still be billions of times cheaper than a single Mantle ' s Approach . Imagine what 49 x 49 x 49 sentinels can do then ! Or 49 x 49 x 49 x 49 ! The flood would be 1 shotted . Also , actually , in Cryptum the Halos did send out massive swarms of Sentinels to protect them from the Forerunner capital ships . They were successful in stalling some time but never in blocking the attacks . So if your theory of exponential sentinel stacking is true , this makes the Mantle ' s approach likely to be capable of destroying multiverses in 1 shot . Most of the time extrapolating beyond the canon is not permitted in these situations . I am not against such extrapolations , if they are founded in reason . Yours are not .