Why does the minimum score only have to be as low as 20k? Also, generally, the more games you play, the more experience you gain (not exp to level up), so score is the best way to sort players. This doesn't mean that everyone will understand the game as well after reaching a certain score, some people just grind AI3 or lower for score. That is then, their problem.
Score and upgrades do not make players skillful, and the lack of score and upgrades does not mean those players lack skill.
That is not the point anyway. The whole idea of skill in elements is mostly absurd to begin with. The point is, BL should be for newer players anyway, to give them a chance to win some prizes too. CL needs to be for those that have played many games and understand the game mechanics better. Do notice how I put skill in quotation marks too.
Don't forget, weekly tourneys are mostly unupped, and those who think they are skilled for joining BL for the different meta game (and it is truly not that different anyway, just slower) well they can join the tourneys for this unupped challenge they seek. Leave the BL for actual beginners. Those with fewer cards and much less games under their belt. And a maximum score would help to regulate this.
Which leads to CL's greater prize. Since CL should be composed of people that have played thousands of games and have a lot of upgraded cards, the minimum score should be considerably higher than 20k anyway. But because these people are more experienced, and have been around longer, and CL being upgraded presents a greater challenge in terms of how to counter super fast decks, the rewards is justified. Where BL should be for actual beginners, their prize shouldn't be that great. Compare to a professional sport, new players compared to veterans are paid substantially less.
What we learned-
1) BL for actual beginners = lower prize
2) CL for veterans = greater prize
If a vet wants to play in BL for the slimmer prize, they should start a new account so they have a small card pool which evens out the playing field for the new players. There is not much point in a player joining BL after like 50 games, with hardly any cards if they are going up against people that own 6 of every unupped card. That's not skill, that is just taking advantage of a system. People should play in the league meant for them.
-edit-
Less reference to skill in elements too, since the game is mostly luck based.