The alternative for your hypothetical plumber to get "on Google" is to buy advertising from them. So instead of gaming SEO, or creating e-zine content that's shareable on the "social" sites, he has to buy ad space from Google so that his site shows up on searches.
Side note. Ideally, your plumber's service shouldn't have to be "googled". It should be in some sort of directory, and we shouldn't be using Google for that. The problem is that people are so very reliant on Google to find everything for them that they don't know any other place to do it. And no reasonable competitor (that I know of) has come in to provide such a directory service.
Most "yellow-page" sites I've seen are giant data-dumps of next-to-useless and out-dated pieces of information. No wonder people "google it".
Minor anecdote. Just the other day, I searched for a known name of a website/company (can't remember). And the first hit was their website, but just above it was an Adword ad for them. Not thinking, I accidentally clicked the adword version. That was $1 right there taken from their advertising budget because of a broken web (I'd argue), and simply given to Google.