A lot of the descriptions of colonies feel like the isolation was recent. Yet what is unsaid in the campaign is the societal impacts of 70 odd years of isolation, that's 2-3 generations of societal decline in either disease, radiation, low birth rate, starvation or - I presume - being collected by the perfected and fed into an Alien hive.
Turning up at a colony that died that way is actually quite a horrific.
It seems like additional homework for GM's to look at each expedition and figure out how those societies declined. For example... none of the original Royal Marines would be active, they would have had to train at least 3 sets of replacements troops to function as a force. It's interesting to consider how they may deviate from the starting point.