in MLS selling them is what you do we are a selling league. If you are keeping a young player its because they arent good enough to make the jump
You can be a selling league but not sell as soon as there is interest the player from Europe. You can hold on to a player to try to win something, if that is what the team is primarily interested in. RBNY is not primarily interested in winning. They are primarily interested in developing players for Liepzig and Salzburg or for sale.
If RBNY's philosophy is to build a team of young players and then sell them as soon as they are ready to make the jump to Europe, the team will never be able to win in MLS because lots of teams have a core of players who are good enough to play in Europe but pay them to stay in MLS. Lodeiro, Zelarayan, Morris, Nagbe, Ruidiaz, etc. all could play in Europe very easily but have spent some part of their prime years in MLS and that has led to winning teams.
RBNY clearly is not interested in bringing in players in their prime of the quality of the ones I have listed. The only way they will be able to win with a strategy of building a core young up and comers is to try to hold on to some of that talent long enough to build a cohesive unit that can compete for 2 or 3 seasons. I am not saying you must try to prevent these guys from ever going to Europe but you certainly have to try to hold on to them until they are 21 or 22, instead of selling as soon as big clubs are interested or Liepzig wants them.
RBNY is likely not going to be a title contender next season, however, if Clark pans out like many people think he can, he could conceivably lead a title contender in 2022 or 2023 but due to the fact that RB corporate does not really care about building a title contender, he will not even get that chance.
The bottomline is that RBNY's transfer policy very clearly is not primarily focused on building a winning team at the moment. If they wanted to build a winner, they would not agree to part with one of their most exciting players a year in advance. They would wait and see.