In my opinion, yes the running scene is in a bit of a decline. I think people are still running however, they are just racing less. There's only so many times you'll be willing to pay to run the same course that you did in the previous year or that you could run on any given weekend for free.
I've done a little bit of anecdotal reviews of some of the local races. In 2014 races were getting anywhere from 25 - 40% better attendance. So a local half marathon might get 500 runners. Now it is getting 300 and sometimes less. The elite races, Boston, New York, Big Sur, Chicago, etc, aren't being hit because those are bucket list races. Your run of the mill local 10K race isn't likely going to be on someone's bucket list.
I've seen the full marathon get particularly hit hard. Many marathons here have a 50% decline in participation.
Some site cost as the driving factor for their lack of racing. I suspect it is mostly market saturation. In Utah here you can pretty much run a half marathon almost ever weekend (and sometimes you'll have your choice between two and three events).
Also I think some of the lack may be driven from the lack of quality. I think some race directors found it to be a good way to make a quick buck. Mismarked courses, long / short courses, faulty timing, etc may be a reason why people are racing less because they've had one too many bad experiences.
What I am amazed about though is the number of women participants. I've been racing for a long time and in the "good old days" it was 66% men, 33% women. That has totally flip flopped with the exception of the full marathon.
Interesting times for sure.