Most people use Gmail because they want to, not because they have to. It's a free, superior product. Pretending voluntary preference is a monopoly is nonsense, but it is a very Mastodon-brained take.
One way monopolies form is by giving away something that others would have to charge money for.
Another way monopolies form is via exclusionary practices and the resulting impression that "things that aren't gmail are less reliable". (Anti-spam does not have to be exclusionary, and anti-spam is generally a good thing, but when it reliably sends smaller providers' mail to spam based solely on them being smaller providers, it is.)
Another way monopolies form is via social effects. "What's your gmail?", or people on first-tier technical support hearing you say an email address and assuming it's a gmail address and having to be corrected, and having never encountered one of those before.
Assuming any of those are "voluntary preference" is a take.
