I always get on physics.se in my free time. I've been doing this for around 2 years now. But I feel like, over the recent weeks, the site has slowed down dramatically. There are fewer people answering questions AND most importantly, most a lot of the questions are duplicates. Is it just me or are we running out of questions?
Much of the data is public, over at Stack Exchange Data Explorer, so you can go and do some number-crunching yourself.
Some of the data is available only to mods, and some is available to >25k rep users, and it shows no such decline. Over the period since January 2015, we have
There are some seasonal variations (such as the obvious dips over the Christmas holidays, and some slower traffic over the summer) and a slow but steady growth. Now, of course, this is an incomplete picture, but the effects you mention should be measurable via the data explorer, so if they're there, you can go and do the number-crunching and convince us that indeed there are some worrisome metrics.
(So much, of course, for the measurable stuff. The intangibles are another matter.)