So, ruin a thing that works for a few people (and arguably was the primary target of the spec before big tech essentially took it over) for no reason other than marginal relying party convenience?

The only thing needed to preserve their utility would be a minimal rewording (e.g. change “preferred” to “possible” or add a “preferred-if-unlimited-storage” for ResidentKeyRequirement), and a requirement of relying parties to add a username field as a fallback.