Awesome, those are exactly the kinds of baselines I was hoping were available.
For the sake of apples-to-apples comparisons, we should look at other games released in the same period, of which there are... Basically none? Huh, it really is the summer doldrums.
Ok, let's see what fits in that window on Steam:
Little King's Story -- some sort of indy game with next to no following.
Peaked at 94 CCU at launch (5 Aug), and dropped to 10% of that on 24 Aug, 19 days later. It dropped to ~22% of peak CCU on Aug 15, 10 days after launch. So, that's comparable, though LKS is also a $20 title.
Telltale's Batman ep 1 launched on 2 August, with 4000 CCU, and had dropped to 10% of that on 6 Aug, 4 days later. These are short, episodic games, though, so let's compare THIS to Telltale's previous launch, Walking Dead, on 23 Feb. WD saw its CCU drop to 10% of its launch peak on 29 Feb, 6 days later. WD ep 2 saw it hit 10% peak CCU in 12 days, while ep 3 also hit 10% in 12 days.
Tales from the Borderlands ep 4 was launched in August 2015, and saw its CCU drop to 10% of its launch peak in 9 days. Ep 5 was launched in October 2015, and saw it drop to 10% its peak in 14 days.
That doesn't look quite as bad for NMS, given that people seem, in general, to be quicker to drop a game in August (or July, for that matter, looking at some other titles) than in other months. I mean, it certainly isn't going to make up for a factor of 3 or 4 in the churn rate compared to Witcher or GTA, but if we assume the summer eats about 1/3 of a game's natural lifespan, then NMS's 2 weeks becomes 3, and while that still doesn't put it in the same realm as GTA 5 or Witcher 3, it really extends that retention curve.
Those games also have several other advantages that haven't been accounted for: They're sequels in popular franchises, they had launches in peak release seasons, they weren't the first PC titles launched by their studios.
That said, there is no fair comparison to make. Hello Games is an indy studio, and they built the game on an indy budet, but it was marketed by Sony as if it were a AAA title. Compared to other indy games, its retention seems to be right on par. Maybe even a little better. Compared to non-blockbuster releases from major publishers, I'm guessing it falls a little short. Compared to other games with its sales numbers?
Yeah, that's definitely where it falls down.