We got Fournier for two 2nds, and he consumed part of the TPE. There is no way to know for sure, but it seems likely that Marcus Smart plus Nesmith plus a 1st pick may have gotten it done, because that's a bit better deal than what they got for him. Perhaps some additional salary filler would have been needed, but no TPE required. I think we would be a better team if that had happened. Getting Fournier didn't modify our capacity to get Gordon. We just didn't do it.
This is saying we would be a better team if we had traded Smart, Nesmith, plus maybe some additional salary for Gordon?
I don't see it the same. We would be worse this season for losing Smart over Gordon and worse in the long run for losing a decent prospect (Nesmith) and a first round pick.
I do agree that the two deals are not necessarily mutually exclusive but the question was are we better for the deal we did for Fournier or would we be better right now if we didn't but did the Gordon deal.
And the thing on the TPE, we will have Bird rights to Fournier so probably we can sign him if we want to. If not, we can sign and trade him and potentially get another TPE back for the trouble. If neither of these two things happen, then we do have the opportunity cost to consider as part of this trade.
I would not trade Smart though just to keep the TPE, and certainly not Smart + Nesmith + 1st Rnd Pick + another player, just to keep the TPE. This is what makes "grading" this trade hard. It is impossible to know what the opportunity cost of the TPE is. It may end up nothing, it may be we miss out on a really useful player.