True.  I think the site overhaul is mostly responsible, and the hacking + this doesn't help.

My gosh though, have you seen Content Paradise in comparison? 

Also, I can't remember, but don't most of the props/scenes at Daz still support Poser mats?  So Poser-only users would still be going there for those. 

One thing I should mention however-- with Rendo's shift towards G2F, of course you're going to see a lot of traffic for Daz, and indeed, from memory, the decline in V4 support does fit that timeline.  So people who want V4 stuff are more and more likely to look elsewhere than Daz or Rendo (such as RDNA-- though to be honest I don't think any other commercial site is continuing much support for V4.)  Dawn and Dusk seem to have pretty good support among the online/freebie community (both on Daz and Poser ends), but outside of Hivewire3D, they receive very little commercial support.  Roxie and Rex get rather limited support on Content Paradise, and practically no support anywhere else, with only marginal support from the online/freebie community.  Even Content Paradise is shifting towards G2F/M, and I believe the diminishing support for any figures aside from Gen2 M/F is the leading factor behind any decline in Poser.  Genesis 1's influence in attracting people to Daz was very limited, but G2F drew a lot of people, including vendors. 

Back on topic however, indeed a combination of bad and inept decisions by the programmers and webmasters is likely behind the Rendo decline.  A lot of people lost faith in the non-forumite staff when it was revealed that the programmers didn't do a lot of the most basic stuff that a programmer should do, such as building the code on a dummy site and creating backups. 
Some people left Renderosity because the new interface required more scrolling, which their arthritic hands could not handle (though it is a blessing for those with vision problems)  The fact of the matter is that the Poser and Daz userbase tends to range in the 40's to 60's range, with many members of the online community using the programs as a hobby now that they're retired/pensioners.

The search function and categories from the site overhaul were difficult to get the hang of, and likely drove people away as well.

The hacking was a major problem as well, due to how long it took for the staff to rectify it.

Now we're seeing the same refusal by the programmers to listen to the customers that we saw during the overhaul-- back then, they initially refused to change the Sale text color due to personal preference.  Now, they're refusing to bring back the three-letter search function, or even so much as giving a warning upon search that words of less than 4 letters are not allowed.