I just think it was entirely too much to give up considering what the going price was for Sergio Santos. Sure, Santos isn't quite as good, but he is still a well above average reliever and he was under control for 4 times as long. He cost much less than Marshall did.
They thought they could extend Marshall. That is known now. It was part of the equation at the time the trade was made, which seems to be the context in which you insist trades be evaluated.

Further, there is little correlation in the trade market between what one GM is willing to give up for one reliever and what another GM is willing to give up for another reliever. Sellers and buyers do not all have the same type or degree of motivation.

Finally, and this remains essential IMO, the value of the players the Reds gave up is based in their evaluations of those players. If they felt the players had limited ceilings, then the trade may have been a simple one for them to agree to. And if those evaluations turn out to be accurate, they'll feel even better about it.

In the end, I score a trade based on what happens afterwards. So far, this one is a big win for the Reds. Could change, but I'm feeling real good about it, even though at the time I was quite sorry to see Sappelt and Torreyes go. Really liked them.