You are right indeed.
If we take into consideration some big AAA games, they all have a decline in the month following their release.
But it’s far from being as quick as NMS.
Let’s stick to the same comparison, which is the date of highest starting peak to the date of the lowest minima having 90% or less of the first peak.
Also, considering the huge amount of sales for NMS, we’ll stick to AAA games that sold well, both on PC and consoles.
First example: The Witcher 3 (Wild Hunt).
Let’s take 24 May (90k+) to roughly end of July, 31 (~9k). That’s more than 2 months to get 90% of decay.
Another example: GTA 5.
From 19 April (364k) to 13 August (39k, let’s be fairplay
). Still more than 2 months to achieve 90% decay.
Ok, let’s take a single player game like the first example: Metal Gear Solid 5.
From 6 September (~90k) to 29 October (~10k). More than a month, not far from 2.
Seeing the chart for No Man’s Sky, one can only wonder why the decay went so fast (2 weeks !!), when all other games “only” got 50% of decay, more or less, for the same amount of time.
I don’t think it’s necessary to go into details like decils, variance and other statistic values, although it could be interesting to have them. I just don’t have the patience to get that done or calculate them 
