Name me one time in the last 20 years when an "average" or "solid" defensive team has won a title. Twice in the last 20 years has a team won the title while finishing outside of the top 10 in defensive PPG and defensive FG.
Miami in 2005-06, who finished 13th in PPG and 8th in FG%. LA in 2001 is the real outlier that finished 23rd in defensive PPG, but still finished 8th in FG%.
Need anymore examples? In fact, most of the teams NBA champs finished in the top 5 in both categories. In other words, you don't simply need average defense to win a title, you need above average if not elite.