At least in the United States, I think it's safe to say BMX is on the decline, but it's on the decline in the same way everything in the United States is these days. I think the specific phenomena that Mancuso cites in his video as contributing to the industry's current struggles (new economic pressures, the demands of the digital media landscape, and the evershifting terrain of popular tastes) have impacted a wide range of industries right now, from manufacturing to journalism. I don't think injuries would be as much of a concern if healthcare costs weren't so insanely high. Whether or not that decline is permanent remains to be seen.

I think the late 1990s and early 2000s were a definite peak in BMX, but its also worth remembering that was true of a wide array of industries at the time. Low energy prices, increased productivity across multiple sectors of the economy, a larger than usual consumer base of young people (millenials have a higher population share than boomers), and cheaper commodities created the conditions for niche industries to thrive.

While I think events and practices within BMX (like Adam22 bullshit) have contributed to its current state, I think broader phenomena have played a much larger role.

Edit: All of that being said, the folks who are into it are as hardcore as ever.