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I think that multiple indications of trust, authority, and user popularity are increasingly being considered to confirm all markup, all links, all content, etc. As Google gains confidence in its ability to make accurate discriminations, it will do so... and low quality sites which don't merit rich snippets are not going to get or retain them.
have those experiencing a slow decline in traffic tried adding new content recently? i had a site in a similar situation until recently and then started adding new and decent content and saw traffic pick back up. i can only assume this triggered some freshness signal?
my view was that i was spending far too much time trying to tweak SEO on my current pages without adding any deceng new content (in fact ive kind of made this a rule for me this year!)
...but it hasn't reversed how Google is seeing your site. Chances are, and this is a guess, that you may have to work on the other onpage and offpage factors that Google wasn't liking.