There is a lot of truth in this...
I think the decline can be attributed to almost everything discussed here... locally we have lost shooters to PRS, in reality at least 10, probably more. I think there is truth that the shotgun does scare new shooters and some others use it as an excuse b/c they either don't like it or they suck at it.
2 gun is easier to get into because everyone has an AR and most have shot some USPSA or that style of pistol match, so its an easy entry.
There have been an abundance of bad stages and matches, its hard to bust on a volunteer but just throwing targets out for a huge round count with no direction isn't always fun. Cost... we all know about that, no explanation needed.
There are shooters who are moving away from the sport for various reasons and not being replaced by new shooters.
I also believe the lack of attendance at a lot of majors (and some locals) can be directly attributed to too many matches. Yes, too many matches are a bad thing, contrary to what everyone said for years....
This all adds up to a sad time for 3gun