Being able to evaluate upcoming changes in ArcGIS Online -- before they are released -- is critical for large organizations that need to coordinate changes with other integrated systems, communicate breaking or important changes to users, update internal documentation, and advocate for newly available innovations.
As it currently stands, access to the new version of ArcGIS Online via the Early Adopters Community (EAC) helps a bit, however, it is a shared environment. That means one cannot customize that environment to match their own organization's configuration, without adversely impacting others.
For example, my ArcGIS Online org is configured to only present our SAML login as the sole option for our organization's login page. If I used my administrative privileges on the EAC instance to configure that to replicate my environment for testing, then no one else would be able to login to the EAC org. Or, if wanted to experiment with SAML groups, I would potentially be blocking others from doing so.
Many institutions -- especially those with large user bases -- maintain a second ArcGIS Online organization for development and testing purposes. It would be very beneficial for such institutions to be able to opt-in their "dev" ArcGIS Online instance to be upgraded prior to an upcoming ArcGIS Online releases. That would enable them to test in a known environment that much more accurately reflects their production ArcGIS Online organization than the EAC organization can.
A pre-release time frame for upgrading such dev instances, similar to what happens with the EAC ArcGIS Online organization at each release, would be fine.