As far as I understood the argument back then was: We need to support a
widely supported mechanism because 'weak' channel binding is better than no
channel binding. This still holds true and is also kinda self evident.
What has changed in the 5 years since we discussed this is a perception of
what that mechanism should be.
The alternative to forever enshrining 'endpoint' as a MUST would simply be
to discuss - in non normative language - the trade off between supporting
something very widely implementable like endpoint and something like
exporter.