Let's say Google entered a clearly different market, such as making & delivering pizza. Further say: Google's pizza is slightly more expensive and somewhat less good than pizza created by their competitors. Based on that, we would ordinarily guess that their market share will be low to nonexistent.

But let's say Google reserves the most prominent position on their SERP for hundreds of relevant queries for one-click pizza delivery. Maybe the majority of people will still click through to Goat Hill Pizza or whatever, with tastier, cheaper pizza. But Google's more prominent position gives them an advantage vs. their competitor, and that will lead to increased and probably significant market share.

Now this is the key: we users could decide that Google's promotion of bad pizza on their SERP means we should switch to Bing. But Google's search engine is, despite this one bad case, still far superior to Bing's. So according to standard micro-economic analysis, pretty much all of us stick with Google, so Google Pizza continues to get the boost.

In summary, in this hypothetical, Google is using the "excess" excellence of their search engine to increase market share of very different products. Since those products may be unusually profitable, they also have an increased ability to drive competitors out of the market. If unchecked, wealthy firms with one great product can use this technique to extend their monopoly into more and more fields. The end result is that more consumers end up with worse quality products and (over time) fewer options due to less competition.

(Note: I'm not arguing that this is truly happening in real life. I'm trying to explain how even a free product with monopolistic dominance can be used to take over other markets with inferior products.)

I can't speak for the down-voters, but I disagree with you on the grounds that Google is not free. You seem to be confusing its users with its customers. The advertisers are the customers. The have terrible customer service for people who don't spend a bundle, you can point out faults with their penalties and get a response of "meh" (when you eventually do get a response). In principle I can see great harm from a Google monopoly, not the least harmed being myself as a small G customer.

That said, I can't argue this article specifically because I didn't rtfa, and probably won't until much later if at all.

I think the consumer are the entities (Fujitsu, Coke, eBay, etc.) advertising on Google, not the people using Google (mom, sister, dad) for search.

From that perspective, Google being the pre-eminent search provider (i.e, where the advertisers' customers search for a product or service) this becomes relevant.

Basically, yes, an advertiser could only advertise on Yahoo! (but in the process lose quite a chunk of their potential customer base).

As a general rule (ie I'm not taking sides here) - it makes it more difficult for higher quality options to compete.

Yelp cant build google to in order to funnel traffic into their local business ratings website

That is a naive view... anti-trust exists to take choice away from the consumer and put it in the hands of the government, thus making companies more beholden to them.