Esri talks about ArcGIS Online being a true SaaS version of ArcGIS Enterprise. I think into todays world this would be fantastic if it were. The problem being is that you still require a certain number of services to be hosted on prem. For example the ability to create custom geoprocessing tools. You are required to host these on your own environments then access them by adding them into your ArcGIS Online environment.
What should happen is that when you create your ArcGIS Online Instance for your organisation you should have the ability to post custom geoprocessing tools in that environment. Similar to the ability to host your organsiations data in the cloud.
