I like your thought process and analysis! I have actually been debating whether to buy DEC recently, for some of the reasons you outlined. One thing that I got slightly confused by is that it seems like your calculation counts the SPS purchased in the SPS purchase option, but it doesn't factor in having the DEC at the end of the DEC route (though it is possible that you mentioned it and I just overlooked it). Figuring out the value of DEC at the end of the airdrop period would be tricky though since who know where the price will go.

Your article makes some great points - in particular, the return from directly buying SPS is much more predictable because you aren't exposed to the downside risk of DEC dropping in value, and also that SPS airdrops are going to decrease when Chaos Legion comes out and the total number of airdrop points increases dramatically.