Because it’s only “eradicated” in New Zealand so long as they wish to remain shut off from the rest of the world.
Trade and communications are running normally. Outgoing international travel is allowed, but of course hardly anyone wants to until the overseas COVID situation settles down. Inbound international travel is bottlenecked on the quarantine system while NZers overseas keep returning here, but that won't last forever because there is a finite number of them, so at some point non-citizens/residents will be able to travel here with a 14-day quarantine requirement and an associated cost (about $4K each). That will work for students, immigrants and long-term visitors. Short-term international tourists are really the only group that will be effectively blocked long term if we don't get a vaccine.
Somewhere in the middle things would get iffy and ethically interesting. The front-line medical worker is probably well-compensated. The essential workers at the grocery stores and warehouses are not.
Hopefully the vaccines are safe and effective and all of this remains a hypothetical.