How many people (outside of the HN circle) actually use Gmail for personal email now? It's mostly been killed by Facebook, WhatsApp or their alternatives.
And most of these alternatives still need an E-Mail to sign up, which loops back to gmail. Statista puts the number of its users at 1.5 billion in 2018[0]. There are some alternatives, yes, but if you ever tried to run your own mailserver you'll quickly find out that gmail users not receiving your mail is very much a problem.
Fastmail? Protonmail? Others? It's not as though the only options are gmail and running your own mail server. They aren't free, but neither is your time to run a mail server and gmail is only gratis, not libre.
I've never said its the only option. I use a rather unknown hoster myself. But the majority of the users are with very few companies; for the US and EU gmail probably dominates the private user market.
Or, let's say it this way: If someone selfhosting his mail server doesn't receive your mail, it's his problem. If gmail users don't receive your mail, it's your problem. And that's not a sign of decentralisation.
[0] https://www.statista.com/statistics/432390/active-gmail-user...