Then this could not be the reason they won—initially it was not the “ubiquity of gmail” as it was just new to the market and could not be synonymous with email. They won by providing a good service for free with an ever-increasing capacity and iterating over features. Note that I’m not suggesting it was perfect or the best solution; plenty of questionable decisions made along the way (the way inbox was handled comes to mind)

That is the argument, that for a time it was in practice a default choice, just a bit before that the default choice was hotmail, at least in italy.

The problem (if we're assuming it was one) still existed in the initial days of Gmail. Point still stands.

...but gmail isn't that ubiquitous! A large majority of the emails I receive are from non-gmail addresses. True, most of these are related to where people work, but still, it drives decentralization, and I think most people understand that email ≠ gmail.

Many of those mails from non Google domains are still being processed by Gmail under the hood. Last I checked, I think Google was processing something like 2/3 of all email.

Really! I'm surprised, Exchange in particular seems to be pretty ubiquitous in the business world.

Not to mention, I do know a fair number of people with non-gmail personal addresses—iCloud, Outlook.com, Yahoo etc.

Hm, on second thought I just did a quick search and I think I was wrong and it's maybe more like in the range of 1/4 to 1/3. Also, am thinking there may not even be a way to measure this very easily outside of just random surveys or trying to gather data separately from major providers.