So Diego wasted his career because he's a hyper-aggressive striker and spent a few years focusing more on that to make his game more well-rounded?
Someone should tell Carlos Condit and Robbie Lawler that hyper-aggressive striking will get you nowhere, and Werdum that focusing more on your striking than your grappling won't lead you to a championship.
I also like the beer-cooler psychology people have in here already. "He just wanted to prove how manly he was, and he gave up on getting better."
Diego's striking may have become one-note in his left-straight/right-straight/left-straight/right-straight combinations that he relied on, and yeah, maybe after 10 years of hindsight on our side we can see he could've won a couple more of his fights if he'd chosen to grapple more in some of his fights than strike, but that's a far cry from saying he wasted his prime by trying to strike more and making baseless accusations about his psychological state.
His striking also led to victories over Luigi Fioravanti, Joe Riggs, Joe Stevenson, and Clay Guida, all of whom were in their primes at the time, and his entertaining performances made him enough money to comfortably set his life up even if he didn't win a championship, so it wasn't really a bad path, even if he may never win a UFC title and spent a longer-than-preferred time outside of the top-10. He also reverted to his grappling to defeat Paulo Thiago (in Thiago's prime, no less, when he was extremely good) and even against Gomi (the 2012 robbery of the year it may have been), so he didn't just forget about his grappling like people think.
A big thing about Diego, too: he used his wrestling as his primary weapon against Jon Fitch and it didn't work out. He spent the entire fight trying to get a takedown and they were either all stuffed, Fitch popped back up right afterwards, or Fitch reversed it and either took Diego's back or held him on the ground while landing soft (but consistent) punches.
It wasn't until after that when striker-Diego appeared.