The word that carries the most weight is left undefined
The verse identifies its audience with two precise qualifiers — those who love God, those called according to his purpose — but leaves the destination of all that working, 'for good,' completely unspecified.
A contract that carefully names the parties but leaves the key term undefined is unusual. You would expect the precision to fall where the stakes are highest.
This verse moves in the other direction. It qualifies its audience twice: those who love God, those called according to his purpose. Two distinct filters, stacked. The reader knows exactly who is in view.
But then the verse arrives at the thing all that working is aimed at, and stops short. 'For good.' Good by whose measure? Good over what span of time? Good in what form — relief, or character, or something not yet nameable? The verse does not say. The word that carries the promise is the one word that receives no further treatment.
The two qualifiers do real work. 'For good' simply stands there, holding the weight.