I think that both systems (the current one and the previous one) have pros and cons.
As for players leaving, in my opinion the main reason is not the league or non-league system, but the fact that there is no ranked play that allows humans to play with wild cards only against humans.
In my guild, there were 19 active players, but now there are only three of us still active, and and the others has left Splinterlands for this reason: Why would a human player who loves Splinterlands and doesn't enjoy playing against bots buy Rebellion and Conclave Arcana cards that, after one or two rotations, they will only be able to use by paying for the Wild Pass to play against bots?
For me, this is the big problem that needs to be solved for acctracts new players, real human players...