Although I like how you think in terms of free market, and would follow the same logic in life, it can't apply to splinterlands.
Leaving the system open just allows grinders to invest very little, get huge rewards, and dump them all on the market.
Other players who are less active or invested a lot have now a bad ROI. You could say they just need to play more, but again, this isn't free market capitalism, it's just a game where you also want lazy people to have fun.
Without them there will be a big loser that will eventually leave.
As for other solutions that achieve the same result, I never heard any. It is definitely possible that there is a better solution, feel free to explain if you want.