on the surface you're 100% correct, but i don't know...
looking at nunez's stats (i'm not really familiar with him, so i'm scouting a line...) he's running a wRC+ of 101, which tells me he basically an average bat, nothing more nothing less. the red sox on the season have put up a wRC+ of 53 from the 3B position, so 53 upgraded to 101 is pretty tremendous for them.
his defense is graded out as a negative according to fangraphs, a -2.0 to be specific, while the red sox are a +3.7 on the season. i don't put a lot of faith into defensive metrics, but it's possible they're trading offensive growth for defensive loss. it's questionable that he's bad, but at least possible.
all of that to say, if devers turned out to be even league average, would they have done any worse with him? maybe he's not league average this year, we don't know, but i would like to have found out, like most fans. i basically think the red sox were afraid to find out while in the middle of a playoff run, so they hedged their bets and brought in a completely average player instead of banking on upside+ from a rookie. if devers starts off hot, i can't see nunez keeping him on the bench or in AAA. to me, nunez is an insurance policy that may or may not be needed. they gave up very little for him.. if it was wasted, it was worth it.