You can burn SPS for DEC if you like but that's not the only mechanism. Also worth pointing out that while DEC price is pegged to the dollar when purchasing things from Splinterlands, transactions between players still use the prevalent market rate at the time.
The DEC peg has been described as being a "psychological peg". It will take some time before it actually gets to the desired rate
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In many instances the demand to buy things from the shop will dwindle as players trade with themselves. That hurts the peg quite a bit. Also small network is small. The required network effect is yet to be seen.