The RA Referral is a very "English" way of broadening membership amongst "the right people". The original demographic of the RA was especially high end. He or she would have spent 50 nights or so at IC branded properties, be a senior corporate executive or manager, diplomat or senior public servant, consultant and the like. Obviously, having to ability to invite someone of a similar demographic was similar to inviting a colleague or friend to join your club. Outsiders were only occassionally considered for membership.
About two years ago, the original SixContinentsClub morphed into the Ambassador program, and the top two tiers of the SCC became the new Royal Ambassador tier. Admission was lowered to a requirement of three stays at three different IC branded properties in a year, along with some PC elite status being earned in the same period. And the referral program was continued. So, yes, the numbers have grown, and I guess with that the exclusivity.
And as we have witnessed here, perhaps somewhat dubious elevations by purchase or gifting. This broadening has certainly changed the original demographic of RAs, but that is part of the strategy IC has played to increase their business and try to lure higher-paying guests from other upper range hotel groups and programs.