From an absolute security perspective, I'm not sure I see how RDAP is much better than a DNS/DNSSEC-based solution, but if you can make RDAP work, you can get most (maybe more!) of the purported benefit of DNSSEC (vis a vis the WebPKI) with a tiny number of deployments, compared to the billions (I think, if you do the math over, say, 10 years, assuming wide deployment --- which won't happen, but, arguendo --- you can get there) we'll spend on DNSSEC...