I didn't make that argument (about the Devils), but I understand the logic behind it. If you're a team that has missed the playoffs for 4 straight years (VAN), and have had the worst record over that span, probably not a good idea to trade your best future asset. Agreed? Better to be conservative and deal depth futures for a player that may have great upside (NJ).
Anyway, it's about process and result. It's both, and it's a balance. A stupid move could yield a good result. A seemingly good move could yield a bad result. In order to judge the trade overall, I believe you should evaluate the beginning given the information at the time. Then, evaluate the end given an appropriate sample. Finally, balance the two to come to a determination. Do you think 2 months is a sufficient sample?
Now, regardless of the strawmen rhetoric or the weak arguments for Benning listed here, with the same backers present (I know you're not of that ilk), that initial read on the deal doesn't change. Miller was a 52~ point forward that was cap-squeezed out of TBay. Benning gave up a future 1st rounder for him knowing that his team had been in the basement for 4 years. That's why this deal was criticized, both here and in the media/twitter. Completely fair.
If we accrue a large enough sample where Miller has proven to be a PPG~ player. Stabilized at this number, then perceptions on the overall balance of that trade will begin to change. But realize, they will begin to change to the positive because the initial perception was so negative. A negative process first followed by a potentially great result.