I think due in part to the fact that I enjoy writing about semi popular subjects which means my content ultimately ends up at the back of the bus. Don't blame me for creating content that I care about please.
If the fact that you write about "semi-popular products" is the cause of your difficulties, doesn't that mean your problem is audience size as much as it is shrinking SERPs?
Granted, if fewer members of a limited target audience see the organic results because of Google's cluttered SERPs, that's going to mean less traffic. But perhaps the solution is to increase your potential reach by covering those semi-popular products in even more depth and breadth. Building the "go to" site in your niche could help you stand out from your competitors, and it could also help your rankings.