If you take away DEC from battle earnings and reward chests there is no more inflow. Giving out 7.5M SPS each month will just shift the problem.
Yes, you can convert SPS to credits to spend them on the market and the seller is then given DEC (so there's your inflow again) and we got a SPS-credits-DEC symbiotic lifecycle. This just makes three currencies depending on each other and makes the market for these currencies more vulnerable and less stable in my oppinion.
In addition the outflow of DEC through guild buildings and market fees is unchanged.
There is your bottleneck.

You just wrecked the price of SPS.

It was a future hope token to be 10 times more valuable than DEC when it was introduced and instead it is now replacing DEC?

Yes, changes need to be made to the ecosystem to get it more stable but this is a slap in the face of everyone trusting the initial intention of SPS and investing in it and its hope/future promise.