This proposal makes cards more demanding, but removes one important strategy.
Using low-level cards is kind of strategy to win. Low-level cards are stronger in some different rule-sets, especially Reverse Speed.
Sometimes we also want to see the underdog's win and their proper reward.

How about removing 50% (or proper percent) of Liquidity Pool DEC rewards and giving them to Splinterlands Cards holders?
Splinterlands used to give SPS to card holders for a year.

Currently, selling cards and putting coins into LPs is much more profitable than playing with their own cards.
Players who own their cards should be rewarded but not much, so they have to play for more profit.
Winning Ranked battles with their own cards should be rewarded more than just putting coins into Liquidity Pools.

Because SPS is fixed and planned to be rewarded, DEC rewards to card holders can be a good solution.
This makes players to have more cards and doesn't destroy low-level cards' potential power.