Hello,
Upon reading the new developer blog regarding the update that's coming, I strongly disagree with the removal of the rookie tier and firmly believe that, despite understanding the goal of the change, the new system will hurt the game's player count on the long run.
Reaching level 10 is quite the tedious task, even with daily league racing, it could take up to 2 and a half seasons (roughly a month and a half/2 months) to reach that landmark, and unless the new Sprint Races payout more than the 10XP that quick racing currently rewards, the majority of newcomers won't stay around until they make it to level 10, for a couple of reasons, the first being that the gameplay loop will get repetitive quite quickly, and the second that there isn't an immediate end goal, with all races entered being meaningless, serving only as a way to level up. The best way that I can put it is that it would be the same as buying any of the F1 games and being forced to complete a certain number of quick races before unlocking the rest of the game, it just wouldn't be fun.
Therefore, the suggestion I'd like to give is that it would be best to leave the rookie tier, however, the leagues would be locked until the player reached level 5. That way, the players would still have to join sprint races to test the game and level up initially, and upon reaching level 5, they would have the choice to join a league and continue playing or, if it wasn't a game to their taste, they could just uninstall the game and call it a day.
I strongly believe this would be much more sustainable for the game on the long run as, not only would it fix the issue of leagues being populated by inactive players, as brand new accounts couldn't join leagues, but it also wouldn't be too demanding for new players to reach the point where they unlock leagues.
This way, players below level 10, currently still competing in the rookie tier, would also not be affected and forced to grind to rejoin the league.