MongoDB Realm is perfect for SaaS products but the hard part is always implementing recurring billing for customers, we would need to write and implement a system like this ourself and maintain it which costs time and money or use third party services like Chargebee or Recurly. Firebase already has a nice extension market where you can install the Stripe extension and you are ready to go and charge your Customers monthly, I would like to see the same in MongoDB Realm with Stripe & PayPal & maybe more, best would be as an official product of the MongoDB product family like Charts or Realm are, just call it MongoDB Realm Subscription Payments or something :)