I think it is still a good idea to initiate the U21 next season. Most countries (50 of 74) will be eliminated from the World Cup by the end of this season. 12 of the remaining 24 will be gone early in the season (after 3 games).
So instead of playing friendlies for a season waiting for the next qualification to start, the players can switch over to the U21 for qualfication. And if you like, simply make players under 22 ineligible for the NT when they are in a friendly season.
The teams that are competing in the initial NT WC should have enough depth to field two young squads, and you can simply give the team in the more important competion priority (so players could be moved to the NT. The two managers should be able to coordinate this so it doesn't happen 1/2 hour before tip-off of a qualification game.