IMO, they'd already done a wonderful job of expanding on their characters just by playing the game. Taako's (literal!) date with Death (okay, someone working for Death, but still). Merle's hidden children, and implied romantic interest with, uh, what's her face, the leader-lady. Magnus' budding friendship with that rogue lady whose name I can't remember.
I've very rarely played a tabletop game where the initial backstories were the beginning and end of a character's life. Players let these creations breathe and grow as the game goes on, and they become more than what they were. They did this. They were doing this, still.
They didn't NEED these new memories. Taako didn't need a new sister. Davenport didn't need to be a non-joke. To me, it reeks of when so many comedy shows/webcomics/whatever try to take a serious turn out of nowhere, and it works for some, but it fails for so many others. I wouldn't say TAZ failed completely, but I would say it was significantly less graceful than it should have been. (I also don't think Griffin's world-building in TAZ is good, but that's another discussion entirely, probably.)
This comes from a place of love. I'm a huge fan of the McElroys! I'm very very disappointed with TAZ's path, of late. ):