First off, Newell explained the reasoning behind Valve's decision to introduce paid mods to the Steam workshop.
He also clarified that the 75/25 revenue split for modders in The Elder Scrolls V: Skyrim isn't decided by Valve. Game developers set the percentages on a game-by-game basis.
Newell sees a bright future for paid mods, though he mentions the importance of data as well when it comes to new programs on Steam.
"Our goal is to make modding better for the authors and gamers," he said. "If something doesn't help with that, it will get dumped. Right now I'm more optimistic that this will be a win for authors and gamers, but we are always going to be data driven."
The entire AMA is worth a read, though some of Newell's comments don't show up because of downvotes. It'll be interesting to see how the perception changes in the next few months as more games introduce paid mod support on Steam.