10 years ago Poser was pretty much the only game in town for the hobbyist 3D artist. Blender was barely getting off the ground, there was no DAZ Studio, there were no free game engines to build your own game or virtual reality.

Today all of those options exist for the hobbyist and freelancer, and Poser is just one of many. Free game engines have become common place and some of the leading names - Unity and Unreal being two of them - have market places that have built up around them whose prices are often in line with Poser and DS content prices, and don't come with the $500 to $2000 licensing that DAZ content requires in order to use in game engines. 

Game engines are used for more than just games. Many people use them to create cg movies, presentations, web comics, etc. Everything you can do in Poser can be done in Unity or Unreal and the learning curve isn't much steeper than Poser since a lot of it is node based "plug-in-play" so to speak, you don't have to know coding to work in a game engine (of course coding helps to customize whatever you're doing). Some engines have modeling tools built into them so you don't even need to invest in external modeling apps. 

This is one of the fastest growing industries out there and there's no sign of it slowing down or going away any time soon. With the introduction of Oculus Rift, and other consumer affordable VR simulators to follow it, I'm sure we're going to see an increase in demand for CG content, especially in the adult oriented areas. 

So IMO, the suspected decline in Poser use is really just a reflection of how many more options there are for people out there today as opposed to what it was just a few years ago.