I believe that it has ALREADY declined over just the last 4-5 years, but I also believe that it's all relative. There was a period when the .40cal was -THE- hot and popular ticket and it distanced 9mm & .45. Those days are now solidly behind us.
However, .40 S&W is still far ahead of something like .38 Special, and when I started shooting (mid to late 1980s), there was nothing hotter in ammunition, sales, shooting, loading, brass, components and across the board... than .38 Special.
We have quite an array of different calibers and flavors and such in the market. I think it's safe to say that .40 S&W has fallen from it's peak already and will likely never reach it's previous heights, but to even half-jokingly mention it in the same kind of popularity arena as .38 Super is not at all accurate, in my opinion.
There's also the beginning of former duty/service weapons hitting the secondary market, a lot of them in .40cal, and that will spark it's popularity over the next decade-plus.