You can't separate out the pieces of the Pierce/KG trade like that; your assumption is invalid. Who's to say that Brooklyn didn't value Pierce as being worth two late 1st rounders? And you've got to factor in the money that Brooklyn committed to paying a decaying KG (you think they're happy he picked up his player option?) Or maybe Brooklyn didn't value KG at all on his current deal, so in effect they gave up two first rounders to get rid of Wallace.
You just don't know what Brooklyn and Boston were thinking about the individual pieces of the trade, so your assumption doesn't really have anything to support it.
What we do know, though... when was the last time that a basically worthless player, a guy earning $10M or $12M - a guy like of like Crash Wallace, when was the last time a team was able to unload a player like that and it only cost them a late 1st rounder? A pure salary dump for a crappy draft pick?
A pick in the 20s simply is not worth $10M.