> 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.