Here's a challenge: walk into a store and attempt to buy a smartphone that is not iPhone or Android.
This is the situation that consumers face. Some alternatives exist, but most consumers are completely unaware of them, because the alternatives have no advertising budget or retail presence.
I think it's quite similar to the political duopoly. Third parties exist, but they have no advertising budget, and moreover, in a Catch-22 situation, they get little or no news coverage, precisely because they have no advertising budget, and thus the news media considers them "not viable." That's a self-fulfilling prophesy. Actually the same situation exists in tech: Apple and Google get huge amounts of free news coverage in addition to their paid advertising. The media appears to feel no obligation to help people escape from duopolies; guess who pays for their advertising...