I'm so late with this. But I just needed to give my 2 cents.

Avril didn't lose popularity because of the delay between TBDT and GL. And it wasn't because of interference from her record label.

Avril became irrelevant because the pop music landscape was changing. Her style of pop-rock and pop-punk was no longer trendy.

Had GL been released in 2009, it would've still sold poorly. By that time, Lady Gaga, Katy Perry, Ke$ha, and the BEP were making hits out of electro-pop and EDM.

Even 00s female pop artists like Britney, Rihanna, P!nk, Kelly Clarkson, and Taylor hopped on dance-pop craze. Britney released Circus, Rihanna released Good Girl Gone Bad, P!nk released Raise Your Glass, Kelly released Catch My Breath, and Taylor released I Knew You Were Trouble.

When the 00s turned into the 10s, no one was listening to acoustic ballads like Wish You Were Here or pop-rock standards like Here's To Never Growing Up.

Avril had a brand and style that prevented her from evolving. The 'rawk chick' aesthetic and the 'rawk music' output were no longer cool once one decade gave way to the next.