Assuming, arguendo, that's even true (and look at the UK....that's simply not true...yeah I know you are talking about numbers in the high 90s which don't exist anywhere except in very well off, not very dense, very few children areas), full world vaccination isn't available until 2023 and people will still move around, seeding new outbreaks (since the vaccine is not full proof). Unless you plan to shut down all world travel substantially, a "fully vaccinated adult population" is an illusion itself.
United Kingdom Coronavirus update with statistics and graphs: total and new cases, deaths per day, mortality and recovery rates, current active cases, recoveries, trends and timeline.
www.worldometers.info