I thought there would be others that would have already jumped into the discussion, but I'll add some more comments, from a long time ArcGIS Server user/administrator and shorter ArcGIS Online user/administrator (a few years) and a new (testing the waters of) Portal user/administrator. ...sorry and warning, I tend to get long winded.
How you might answer this question really depends on what is is you are trying to do, and what resources you have (hardware, cloud, staff), etc. Security and the ability to use the ArcGIS Online platform might also influence what you can use. For some, the in-house Portal is very similar to ArcGIS Online, but on-site vs in-the-cloud. I'm simplifying this a lot, and I've been told that calling Portal the on-site ArcGIS Online is not the message esri is trying to get across, but in my mind, that is still the easiest way to explain it.
Now, as a Department that has access to all three (plus OpenData and other ArcGIS Server features...not called extensions anymore) which now fall under the ArcGIS for Enterprise (AE) umbrella, we tend to use a combination of the items. Personally, if I had to pick just one of the tools from the AE set, I would still pick ArcGIS Server (and Desktop of course), but I don't think AGOL/Portal will replace ArcGIS Server, my opinion. ..Also, besides the names users, there is still a credit cost for some items/processes (but not all).
For one, although improvements have been made with the Level 1/2 named users, Your Guide to Named User Levels | ArcGIS Blog and we really don't have pricing yet, chances are shop will not provide a named user for every employee (at least not at this point). So, having a way to allow internal only, secure services to all, without requiring a name user, is something that I'm hoping to work out with a combination of ArcGIS Server and Portal. Time will tell if it works as I perceive it.
As I mentioned, jumping on the Portal band wagon is new for me, but this is how I see all tools working for us. Let me state that we do have services that are "secure" but we provide these services output thru Javascript and other web tools. But we are not as limited as some agencies that have no access to the internet. Also, I am no expert and I'm learning navigating all these systems, especially with the new releases, just the same as everyone else.
My concept/opinion:
- ArcGIS Server will continue to provide services, both public and secure. It will also be the choice for creating custom geoprocessing services since many rely on Desktop.
- JavaScript and web apps like Web AppBuilder will access items using a proxy (in particular for secrue services)
- Some AGS services will be "advertised" on AGOL. This works well for public services, not so well for secure, at least in our setup.
- Portal will be installed onsite. Named users and groups will be used for some itmes. For secure services, I think we will have better luck setting up a proxy with ArcGIS Server and Portal being on the same internal network. With Portal's webadaptor, I think we should still be able to expose some Portal items to "the public", while keeping other things totally locked inside the firewall. This may not be possible, but I'm hoping.
- ArcGIS Online, on the other hand, is a great place to "advertise" services and downloads for the public. It is also accessible for those outside our firewall for the various apps available, including the need for named users for security. And it is also well integrated with the OpenData platform (not sure how Portal plays with that).
So, after this long winded rant, I do not see ArcGIS Online (or Portal) replacing ArcGIS Server. For some, maybe, but for the bigger organizations, not anytime soon.
Anyone else want to weigh in?
