I guess part of their counterpoint would be that 'onboarding' is a narrow subset of usage and only requires that new users be given a small RC allowance to be able to interact. RC pools can do that painlessly to some extent, especially given that in practice many new accounts go inactive and then use literally zero RCs, as well as zero resources aside from the ongoing consensus state for the account itself (under the current design these are never cleared out).

I of course share the concern that focus on RCs seems to distract from the core issue of physical scalability (see my similar comment).