Another kick the can down the road solution. You can't issue sps as rewards for eternity because sps has a limited supply. Even with the decay rate, lots of luck trying to sell more cards to an oversaturated market with diminishing returns.
The game needs a tradable energy token that is issued based on collection power and/or staked sps. An infinite amount of energy can be issued and it will be burnt for eternity every time a battle is played.
The emmission rate of energy can be regulated so that it matches the effective amount of games being played and easily adjusted periodically to match the demand.
It could also generate revenue by selling energy if demand is high for energy, or save money by buying back the energy at below server costs.
An ingame dec/energy pool would also lock up a lot of dec.
And energy would be a tangible reward determined by market price.