Number of people using Gmail is basically 100%

Your implausible estimate got me curious; so I checked[1,2] the first couple of result from Google on email marketshare. GMail is 27.7%. Apple has double the share, at 54%, you should be worried about Apple instead.

1. https://www.litmus.com/email-client-market-share

2. https://www.oberlo.com/statistics/most-used-email-clients

That's unbelievably high for Apple when its hardware market share is much less than that (around 30% for iphones and significantly less than 20% for laptops and desktops, hard to quickly find consistent numbers).

When I look at the email addresses in the various communities I belong to, it’s at least 80% gmail, and the rest are mostly defunct services are just barely hanging on. Yes, I run my own email server, have for years.

> the rest are mostly defunct services are just barely hanging on.

...and what a miserable bunch most of them were: 2 MB attachment limits and 10 MB mailbox quotas?! GMail had to drag all of them kicking and screaming into the 21st century when they debuted with a 1 GB mailbox quota.

That’s clients.

People add their Gmail account to their iPhone and get Apple Mail.

Virtually all email goes to Gmail between personal accounts and GSuite stuff.