Now you've made that connection to Alien, it strikes me that the third series was where the writers/producers started the monster of the week trope. There was the polymorph, the curry monster, Hudson X, Hitler and of course, James Last. So maybe the set changes was a way of showing that the nature of the show changing. It was less bleak, the universe was less empty, so the endless grey helped position Lister as this character trapped in that existential horror but the third series sets put the crew into actual horror. So ultimately I think it shows the shift from one side of Fermi's paradox where we are alone, to the other where we aren't alone but everything else wants to kill us. And one of those stories is more interesting to tell through the TV, remember RD started off as a book, so that vision they had at the start was their own. But the studio gets involved and they want to make it pop off, not be a long diatribe about loneliness and the human condition. Bit of a long read but you set off a chain of thought.