It’s certainly a worry, but at least other options do exist. And if you own a domain name, you can move your email hosting (including all your addresses) to the provider you like the most.
And it’s certainly a much better situation than other chat systems, where your identity is fundamentally tied to your account on that particular service. You can’t move to signal without moving everyone else to signal at the same time. I only need one email provider and one email app. But for messaging, everyone ends up needing signal, and WhatsApp, and Facebook to talk to each other. Imagine if you needed half a dozen email apps and accounts for each one to use email. “Oh, you’re emailing a company who uses Microsoft? Better open the Microsoft email program and remember your Microsoft login”.
I agree with your point but has this not happened with Gmail, Microsoft and Yahoo! for email anyway?