We're looking at the possibility, within our next multi-year deployment cycle, of positioning AGOL as the center of our Enterprise GIS effort. That isn't to say we won't have multiple ArcGIS Desktop deployments or complex custom web apps, but the idea would be that most of our customers would pivot through if not fully engage AGOL for their GIS needs. As part of AGOL, I also mean esri baked "apps/templates" such as story-maps, operations/executive dashboards, Collector, esri Open Data site, Web App Builder, etc... We would position those "pre-baked" configurable apps higher up on the solution chain and only go down the yucky-bits-road of custom dev when required.
I'm a little (actually a lot) confused about why an org would go with AGOL vs (or alongside?) Portal for ArcGIS so I'll add that to the choices below.
Also, we currently have a very robust internal (but exposed externally) ArcGIS Server setup. The idea would be to keep that as it is and simply point the map/feature services at AGOL for consumptive purposes. Comment below if you feel on-site ArcGIS Server deployments are yesterday's solution. Perhaps that's another poll.
We are a 2,600 square mile county with a population of about 310,000 and employee base of about 1,700 and have been customers of esri for onwards of 20 years.