Both Aqua/Magma and Galactic were way closer to their goals than Ghetsis was. Ghetsis wanted domination of the world/Unova. All he ended up doing was having N beat Alder and rallying nearly all of the strongest trainers in the region against him which brought about his fall. Really he had a lot more to do since there was still seriously strong opposition left, more opposition than in Gens 3 and 4.
In Gen 3 Aqua/Magma's goals were to awaken the slumbering Legends, which they did accomplish. The second part was to cause natural disasters to increase the sea or land, both of which were also done. Only reason they stopped was because they couldn't control the legend like they thought they could, and gave up before the climax.
In Gen 4, Cyrus had the legends of Space/Time under his complete control and was creating an alternate world in his image. He was stopped just short of attaining his ultimate goal. Like an inch compared to Ghetsis' mile.
If the MC's didn't beat plasma, there was still a lot more work that they had to do. With N being the sole owner of a legendary Dragon he could go about uncontested, but there were still people that didn't agree with the philosophy. Plasma would have to have destroyed the league as a whole and then win the hearts of Unova rather than confuse them. Even after that, once N would have found out the real goal Ghetsis had, I'm sure he would have rebelled against him so he had his own downfall supporting him while his plans were still a secret.
I don't think Ghetsis ever had a chance for complete control, nor a chance to fully win in any way.