You can only do it during growth phases or if there’s complimentary products with margin. The story I was told about Office 365 was the when they were using spinning disk, exchange was IOPS-bound, so they had lots of high volume, low iops storage to offer for SharePoint. Google has a similar story, although neither are really unlimited, but approaching unlimited with for large customers.

Once growth slows, churn eats much of the organic growth and you need to spend money on marketing.