Whatever the name of a team is that comes after the academy but before the first team is not the most important in this discussion.
That you have a team in the age between 18-21 is what counts. This important step, before a player really makes it is as a professional, will give so much to the career of most players. The opportunity to get used to a higher level surrounded by other and older professionals adds a lot of stability, experience, knowledge and confidence to a players final phase of development.
In a known environment (your own club) making your final development and receive a chance to train or play with the main team is what you are aiming for when you start playing football.
Several countries in Western and Eastern Europe had a closed league with only U21 teams that played weekly a match against the same age group (like in the youth leagues). Some countries gave an opportunity to play in a second or a third professional league and to play against adults.
The second option has been copied now by some of the leading football countries, this gave clubs the opportunity to enter a professional league. This in some countries gave such a boost for talented players; that they made a secure step to their first team. Another effect was that those “U21-23 teams” became leaders in the second or third division by playing in the top of the league and at some of them became champion.
For me that is a confirmation that a long-term development plan should not end after u19, but it should still continue for one or two years before a player or players are ready for the highest level.
Exceptions we will always have, players that reach the first team faster but for the majority of our players a reserve(U21-23) team is the best solution for having a chance to become a professional player and reach their personal top!
Should it be a discussion?
Maybe a federation, the clubs and the professional leagues should simply sit together and organize an opportunity, so that talented players get a real chance after years of investing in their football career?