Whilst Google's search is currently better than Bing and DDG, I would argue that some nails in the coffin were Google's ancillary products.

When Gmail was invite-only people couldn't get on there fast enough, mostly so they didn't have to use Hotmail anymore.

But I'm also old enough to remember when the market quite happily supported Google, Lycos, Ask, Wolfram, www.com, Yahoo and my personal favourite, Alta Vista. I didn't really use Google until it was the only one left. Once Gmail came out it was just convenient to stay.

Some of these older brands are still around but are powered in their backend by Bing or Google. Maybe they do have a monopoly....