Actually, Will doesn't simply criticize rappers for bragging about how much money they have. He criticizes rappers for gratuitous expositions of their wealth and the lack of variety and creativity in their discourse: talking about money all the time and the things they have. Also, the "20 mill" line wasn't really bragging. It was more of a "say what you will, but I'm keeping it real and I'm still prosperous. I may not like what you say, but it doesn't seem to be hurting my success."
Will could have responded in a number of ways. He could have mentioned how good his family life is, or how much he enjoys his career, or life, etc... In this case, he had to dumb things down because for a lot of people, the only way to measure success is wealth, so he dropped the 20 mill line. He's done it in the past... the "Microsoft" line for example. However, Will's also made it clear those things he really holds as valuable in his life... he aptly entitled the track "Who Am I."