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.
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.