It's possible that Google is leading me astray since that's typically how I'll find Q&As on there, but I don't see the same issue on the other sites in the Stack network.
In these cases, the technology needs to keep pace with the systems (both computer systems and people systems) that it supports. Failure to do that means that the people who are creating content are having to expend more effort to make up for it.
Be it people leaving, or needing to spend more effort to attain the same quality - the result is the same. The quality of the site (questions and answers) is diminishing as people are less able to provide good answers and curate existing material to make good content more visible.
It's for reasons like this that there are new startups cropping up in the field. One that's exciting to me is Quidio, a bounty-based Q&A marketplace that demolishes the 90-9-1 rule for better quality results.
I am not sure if SO is less useful or just has become less useful _for_me_ over time.