The Blazers traded two highly paid starting quality but not championship players) for the ability to retain the prospect they do value (Simons) going forward - because the organization would not have been able to sign him going forward otherwise. The only difference between what happened and what you identified as a need is that the prospect was already on the roster.
In other words, the organization believes that with opportunity, the prospect they would have lost would provide the production of the assets that were lost.
The reasonable business equation to make is not Roco + Powell > Keon Johnson which is what is happening this year, but ask if Roco + Powell > Simon + Roco - because next year, if the trade was not made, the Blazers would lose Roco anyway and not be able to sign Simons. So, basically, since we can eliminate Roco from both sides of the equation - the question was - is Powel > Simons. Obviously the organization did not think so.
You can argue that better options were available - and maybe they were and maybe they were not - I certainly do not have the data to back this assertion - but your basic formula to asses the trade in a vacuum is not right.
