Whether or not the browser is open-source has no impact on this issue. The monopoly would not exist because Google/Apple/whoever are the only people allowed to

Whether or not the browser is open-source has no impact on this issue. The monopoly would not exist because Google/Apple/whoever are the only people allowed to make browsers. Google would simply have the power to make all of its services (search, docs, etc) totally unusable on any browser that isn't a version of Chrome compiled and distributed by them, thus making all other browsers useless to the vast majority of the population. Because analytics data is money, other companies would have financial incentive to follow suit. By the way, Chrome contains proprietary code. Chrome and Chromium are not identical.