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Did you take into account that (Staked) SPS is a depreciating asset (Voucher amounts and value as well as interest are going down) while Liquidity Pool SPS is an appreciating asset?

In that sence they are both depreciating given that you get a % ouf of the ever increasing pool.

What I did not take into consideration is the compounding of the daily rewards, but that would quickly become complicated since we would have to make guesses regarding the voucher price. So this calculation should be seen as a snapshot of the numbers if you were to act in that very moment.

Mmmmmm..... no.
DEC will depreciate vs. SPS as the AD value of DEC becomes less and less, as DEC depreciates vs. SPS- shares in the LP will actually appreciate as the SPS:DEC ratio automatically adjusts- multiplying your AD points over time.

Absolutely! it's a snapshot- and yet, Staking is a 4 week commitment. If someone stakes their SPS, today, they can't move it into the pool next week when the ratio changes so you definitely should have considered at least a week of compounding in your analysis.