RA21 doesn't address the yet-another-WAYF problem. Radical inclusiveness would.

The fundamental problem with standards is captured by XKCD 927.

XKCD https://xkcd.com/927/

Single sign-on systems have the same problem. The only way for a single sign-on system to deliver a seamless user experience is to be backed by a federated identity system that encompasses all use cases. For RA-21 to be the single button that works for everyone, it must be radically inclusive. It must accommodate a wide variety of communities and use cases.

Unfortunately, the draft recommended practice betrays no self-awareness about this problem. Mostly, it assumes that there will be a single "access through your institution" button. While it is certainly true that end-users have more success when presented with a primary access method, it's not addressed how  RA-21 might reach that state.

Articulating a radical inclusiveness principle would put the goal of single-button access within reach. Radical inclusiveness means bringing IP-based authentication, anonymous access, and access for walk-ins into the RA-21 tent. Meanwhile the usability and adoption of of SAML-based systems would be improved; service providers who require "end-to-end traceability" could achieve this in the context of their customer agreements; it needn't be a requirement for the system as a whole.

Radical inclusiveness would also broaden the user base and thus financial support for the system as a whole. We can't expect a 100,000 student university library in China to have the same requirements or capabilities as a small hospital in New Jersey or a multinational pharmaceutical company in Switzerland, even though all three might need access to the same research article.

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This is my fourth comment on the RA-21 draft "Recommended Practices for Improved Access toInstitutionally-Provided Information Resources". The official comment period ends Friday. This comment, 57 others, and the add-comment form can be read here. My comments so far are about secure communication channels, potential phishing attacks, and the incompatibility of the recommended technical approach with privacy-enhancing browser features. I'm posting the comments here so you can easily comment. I'll have one more comment, and then a general summary.

Update July 10, 2019:

RA21's official response to this comment is:

RA21 envisages supporting the anonymous and walk-in use cases via federated authentication. It is anticpated that federated authentication and IP authentication will exist side-by-side during a transition phase. The specifics of the User Experience during the transition phase will need to be determined during implementation; however it is likely that the RA21 button will simply not need to be displayed to users who are IP authenticated.

I suppose self-awareness was a big ask. The revised recommendation includes some "envisaging" of use cases that was glaring by omission in the draft recommendation. The added section 2.1.1., Employ appropriate authentication mechanisms for specific use cases, is an improvement on the draft; but the revised recommendation has not retreated from its end-to-end traceability "guiding principle".

RA21 used the same response for a comment by Ohio State's, Jennifer Vinopal:

I want to reiterate a point that a number of commenters have already mentioned: there is no discussion of how public or walk-in (or other unauthenticated/unauthenticating) users will get access to resources through RA21. Public libraries, as well as many college and research libraries, negotiate our e-resource licenses to provide access to walk-in users who aren?t represented in our IdM systems.

Don't forget, EZProxy was supposed to be a transition phase!