Scott Alexander issues an extended rebuttal:
https://www.astralcodexten.com/p/is-there-an-illusion-of-moral-decline
The paper mixes societal intra-societal with things like war - they should be treated as distinct, with the morality in question being more about the former. I've a simple argument to motivate declining morality: as the number of people increases one becomes more anonymous and this anonymity is also enabled by modern means of communication and geographical mobility. The link between anonymity and being nice is well established. For instance, in London, you can be a bit of a jerk to random strangers because of the chances of it coming back to you are hugely lower than in a provincial town - and the bigger London gets the more itinerant its population is the jerkier one can be.
