If you lose the domain, you lose everything (email, web, services, etc.), possibly permanently, with no option to migrate without losing your domain, branding, SEO position, etc.
e.g.
* Your server got hacked, and your entire account gets blocked
* You get a shock bill from AWS, and lose access to the account because the account is unpaid.
* Also it allows your hosting provider to hold you ransom to payment or you lose everything