I have a different idea now that some time has passed - and it is probably, radically different to what you have proposed:

How about we get rid of leaderboard DEC rewards being "new DEC", and do the following with DEC:

2% to Market Fees for SPL Running the API / Market
1% to the Buy Market
1% to the Sell Market
1% To the imaginary, proposed "Leaderboard Reward Pool", distributed as follows:

1. Fill Bronze through to Diamond Leaderboard rewards up to the current cap in both modern and wild as the season progresses. Once these reward pools (from the 1% market fees, are "full")
2. The remainder of the 1% of the reward pool "fees" go to champion (split equally, as the others would be, between wild and modern)
3. Champ rewards (from market fees) are then uncapped, and are based entirely on that 1% of market activity.

In addition, abolish the ability for cards to be listed with less than a 5% market fee.

Pros: No New DEC generated
Cons: Potentially different rewards each season

In summary, for each 1 DEC of market fee:

0.2 to Bronze Until Full
0.2 to Silver Until Full
0.2 to Gold Until Full
0.2 to Diamond Until Full
0.2 to Champ (uncapped)

In the event that there is not enough DEC generated by market fees to fill for the reward pools, SPS is burned to obtain the DEC. DOn't know where that SPS could come from, maybe from the allocation for battle rewards?