"Decay" is a loaded word reflecting the author's bias. All communities change over time as they grow. Their culture then reflects the users. We may not like that community but clearly a lot of people do (after all these are the largest sub reddits we're talking about right?)

This is sort of the "This neighborhood was so great until all the yuppy/hipsters moved in and the art stores were replaced with coffee shops serving $6 lattes" argument.