[Dr Kain]

How would you then have had them destroy the fleet?

That is because Ackdos is an emperor, not a fighter. It carried no weight because the rangers were not going to let him even have the chance to use his powers. I mean seriously. How many times do you want to see the rangers lose against the main villian and then have them get that final energy to defeat him? We know they are going to beat him, so why beat around the bush?

Were you expecting them to keep the keys? They gathered the keys, got what they needed, and then gave them back. What more do you want? It's like you were setting yourself up to be disappointed no matter how it ended. Would you rather it had been like Goseiger where there was no final fight at all, they just had to stop drills from reaching the core?

If you think they did nothing throughout the series, you need to either rewatch the series, or watch DekaRanger and Shinkenger. Nothing happened at all during those series. In fact, the villians had even less development in Shinkenger than Gokaiger. Barizorg had more development than the entire Shinkenger villian cast.