If you're going to hire a plumber, do you care whether or not they can create clever content? Of course not. If you search for "plumbers in kansas city," you want a good plumber in Kansas City. You want someone who shows up on time, does great work, and makes you feel important. The fact that a mediocre plumber can produce clever content and end up ranking well is a flaw in Google's algorithm. Google's real goal is to return the best list of products in the SERPs, regardless of their writing, graphic design or production skills. That's what their users want when performing transactional queries.
This isn't an algorithmic problem that Matt Cutts will be discussing on the Google blog. Google has been pushing content marketing as a good strategy for years because they prefer it to having spammers embarrass them. Now that white hat SEO is ubiquitous, they'll move on to their next paradigm shift.
Google's Content Marketing Solution
Content marketing works because authority gained from superior content passes through to the authority score for a company's core product. Google will simply break this link. They will assign one level of authority for informational queries and another for transactional queries. Websites with strong informational authority could still rank for informational queries, but not as well for transactional queries. Companies (websites) with higher transactional authority (i.e. better product quality) would rank higher for transactional queries. Google's ability to categorize queries is already pretty advanced. It's not much of a leap for them to categorize quality ratings for a website.
This solution would allow a crappy plumber with great linkbait to rank for informational queries like "how to do x," and a good plumber with poor linkbait to rank for transactional queries such as "plumbers in kansas city." Those are good results, and are ultimately what users want.
This distinction between informational and transactional quality could be calculated by analyzing different data types. For example, ratings coming from sites like Yelp tend to speak to the transactional value of a company's products, and links to internal pages from blogs would be more likely to speak to the informational value of a website.
When & Where Content Marketing Will Stay Relevant
Content marketing will always work for publishers that are in the business of providing information. It will also always work as a top of the funnel lead generation strategy for big ticket products and services. These products target prospects with high levels of involvement that are motivated to learn and seek information.
I believe that content marketing will become less and less effective way of ranking for transactional queries.