The fact I use Gmail allows them to make Google maps and Google assistant better products than say bing maps and Cortana, because bing maps and Cortana could never get access to those reservation emails which make the whole thing work.
Such monopolies should be illegal - everything should be sharable via an API. Sadly those seem to have gone out of fashion with things like Cambridge Analytica.
For example, if I extract OGuth2 app information from Thunderbird, then I could theoretically make another user authenticate against Thunderbird's app and then ex-filtrate all of their Gmail data.
Gmail API is blocked for purely political reasons. E.G. real-time delivery of e-mails which is not accessible through non-Gmail clients.