It isn't about market share. It is about consumer choice. Access to google alternatives are available to everyone. Many people don't have a reasonable choice when it comes to broadband.

Economists traditionally analyze market concentration in terms of market share, not the mere existence of choice. I will point out that you felt the need to qualify "choice" with "reasonable"--because Comcast competes with DSL and cellular almost everywhere. But that some qualifier applies to search. Even in markets where Comcast is the only cable provider, it usually doesn't have anything close to 90% market share, because DSL and cellular are a "reasonable choice" for quite a lot of people. (About half of households don't subscribe to any streaming video service.) Thus, judging by market share, Google alternatives are apparently even less viable in the search industry than DSL and cellular are in the home broadband industry.

Competitive markets tend toward monopolies because everyone wants the best product. Monopolies are only prevented by vendor lock in or natural barriers (like geography, which mostly does not exist on the Internet), which is why you have a few pseudo competitors in broadband and cellular.

> Economists traditionally analyze market concentration in terms of market share, not the mere existence of choice.

Actually, many economists - and perhaps most of them - argue that what's fundamental to monopolization concerns is the presence of undue barriers to entry that make the market non-contestable. "Market share" and "choice" are not helpful on their own, because a big market share could be entirely due to the incumbent pursuing efficiency and the consumer's best interests, so as to keep themselves ahead of any possible competitors. But this is a benign dynamic that will be quickly corrected should the incumbent fall behind, as competitors would quickly enter the market.