Most stories of generational ships either have the whole crew in suspended animation or the whole crew living a life within the coocoon of the ship. It would be a bizarre journey to take, to live in one world, and know that your ancestors will live in a new world but you will live and die within a tin can floating through space. In this story the Gaurdians are the humans who are watching over the ship, and they know they have 700 years of journey ahead of them. They will not live through the journey and it is their descendants who will guide the ship to the destination planet, wake up the sleeping humans in storage, and settle on the new world. 700 years is a lot of time and clearly the societies would diverge wildly, with the sleeping human cargo still knowing the society they left behind on their original world, while the Guardians will have had 700 years of societal development.
What's different for them is that they met the Monoids. The Doctor and companions spend the first two episodes with these people at the beginning of their journey. The Guardians are in control, and the Monoids are mute servants. However the Doctor did recognize that the Monoids were very intelligent and capable people. The Doctor and Companions then depart and are shocked to arrive in exactly the same place they departed, in the Ark. However it is the Ark at the end of the journey, 700 years later.
In the intervening years the Monoids took over control. Where the Guardians had been domineering masters, the Monoids were even worse, employing guns freely to keep the Guardians scared into submission.
The Monoids in their voicing reminded me of Daleks and it would have been one or two small steps for them to adopt the EXTERMINATE frenzy of the Daleks. This story also reminds me of the Israeli's, where the Jewish people have this historical memory of having been made a wandering people without a home land of their own and having a second-class-citizen role wherever they went and sometimes facing bloodshed and genocide. But put the Jewish in charge of their own homeland and they became cruel overlords dominating the Palestinians who were formerly the rulers of Palestine. There's something here similar to how someone who is abused during childhood often becomes an abuser themselves when they are in charge. There are always contextual circumstances which lead to the cycle of abuse and domination, especially with the story of Israel that I just oversimplified. And in the case of the Guardians and Monoids we do not see the intervening years, we only see the beginning and the end and the reversal of roles.
I started this with a gripe over continuity. What I mean is their excuse for these people to be on a generation ship. It is said this story happens 10,000,000 years into the future from our present time. And that at that time the Earth is about to be destroyed by an expansion of the Sun. Okaaaay, while Astronomers to believe our Sun will eventually expand and destroy the planets, that will occur 5 billion years from now. However in the 2005 episode "The End of the World", Earth is finally destroyed by the expanding Sun around AD 5,000,000,000. Okay which is it, was the Earth destroyed 10million years from now or 5billion?