is dinghy sailing on the decline?

The attitude of sailors has nothing to do with it. The image of sailing comes from the general media, who (if they cover sailing at all) focus on big glamourous boats with well-funded PR operations which can feed news stories to journalists. Such boats are inevitably (and for the most part realistically) associated with the wealthy. Except for a couple of weeks every four years (guess which), dinghy sailing is of zero interest to the general media - and why should we expect otherwise? How much coverage of other participant sports do you see? 

A well-known market research company provides the statisctics on sports and leisure participipation which a lot of organisations and companies use for planning. I won't name them, to protect the guilty, but I happen to have access to their subscriber-only reports. A while ago I looked in detail at the participation numbers for another sport (not sailing) and they were complete rubbish. Anybody who knew anything about the sport would know they had to be several times higher than reality. You couldn't fit the numbers they were reporting into the facilities available. It turned out that the statistics are mostly based on self-reported surveys, and basically people lie to make themselves sound more interesting. They are actually coach potatoes but say they are out doing this sport and that every month, when the reality is they tried it once 2 years ago.

So don't take any of these statistics at face value.

In 2002 McKinsey did a piece of "pro-bono" strategy work for the RYA (McKinsey are arguably the world's leading management consultancy company). They concluded sailing participation is in marked decline. I can't remember how they reached that view and the report no longer seems to be available on the RYA website. However if anyone has a handle on club memberships, it ought to be the RYA. This is because if you run an RYA-affiliated club, you are supposed to report annual membership numbers - at least that was the case when I used to be secretary of a small club.