I think someone from say MailChimp would be more qualified to answer this question than I would... :)
They make money from sending emails to lists. Very different business model to Gmail or the like.
Charge a monthly / yearly fee?
if you are willing to pay for private email, why not any of the dozens of existing ones? e.g. protonmail or fast mail? there is little differentiation available for them here.
The same way that other legitimate email service providers do: they charge a monthly or yearly fee for service.