The only reason why WHOIS actually needed to stick around for so long is because of thin registries (.com and .net are examples) where the registrar manages the

The only reason why WHOIS actually needed to stick around for so long is because of thin registries (.com and .net are examples) where the registrar manages the contact information for the domain rather than the registry. When you do a domain transfer, the registrars involved are required to get confirmation of the transfer, which means that the gaining registrar needs a way to query the contact information for the domain. With thick registries, all the gaining registrar needs to query this is the EPP key (AKA the transfer code, transfer key, authinfo, &c.) and the domain name, and then they have read-only access to the domain object and any contact object associated with the domain in question. With thin registries, there's no such thing, so the only way to implement ICANN's Inter-Registrar Transfer Policy is to query WHOIS. Up until ICANN finally forced a standard format on gTLD WHOIS responses, this was a nightmare as all too many registries would go out of their way to make it difficult to get any information out of WHOIS. I'm glad to see WHOIS starting to die for domain names and be replaced with RDAP.