For many people email is not really that important. While I agree that it would be better to fully own your email address (domain) it's just not that important. If I don't have access to my email, I'll let my bank know that I use a different mail.

I would pay protonmail 10 bucks a year for my mail address just to support them but they don't offer that. I only need it for sometimes amazon and who knows. But it's certainly not worth 50 bucks a year - unless you own a business that highly depends on it. I don't use it for communication with real people. Just websites that need to prevent spam or want to communicate with me for notifications.

Not everything Free means you're the product, do research on the email provider, do research on the company. Relying on "You're the product" is going to be complicated, also paying for anything doesn't make you less of a product, just look at games.

Set up your own emails or procure an Open-Source provider

if it’s free than your the product

Providing a basic email service with small storage/traffic allowance costs in the order of cents per UserMonth.

From the business perspective, giving something for free can basically be considered a marketing (advertising) expense -- essential for building a successful business.

If you get the free storage/traffic/feature allowance in the sweet spot, it's useful enough that free users stick around and you get a good reputation, yet juuust constrained enough that users who want that little bit more convenience or features will consider a paid plan. And word-of-mouth advertising is super effective -- all the happy users will recommend your paid service to family/friends/acquaintances who happen to be looking for a service provided by them.

Another benefit free services can provide to the business (especially in early stages of developing a system/business) is real-world testing at scale of your infrastructure and processes with real users.