In years past a Cup campaign was always a one off proposition. The whole intent was to win (either challenger or defender). Once the last race was sailed, that was the end. Assets could be held over or stored, but that was usually the end of the programs.
Now there is the need to secure talent in between campaigns because the professional market is open to the highest bidder. In the past, with stricter nationality rules there was no need to have this continuity because with only one or two teams per Nation the talent was there.
The Cup has a history of the nationality rules moving