The New Age stuff apparently had strong trade pre-orders as well, and those surpassed a lot of the established big names. With newer IP's like this I think you have to look beyond the direct market and see how the books are doing in other formats, where the newer fans seem to gravitate to; digital and trade and all that. I have no idea how these books did digitally, but I do know the direct market isnt the sole indicator of success it used to be. If it was, books like Ms. Marvel never would've lasted.
I agree that removing the artists so early was a mistake, though it didn't surprise me; those books were never likely to make it in the direct market and names like Reis and Daniels and Rocafort pull bigger page rates than the other guys (reducing the profit margins) and are always in high demand for bigger, higher selling titles.
And I think it would've worked a bit better if they had included the New Age heroes in Metal itself, with a role in the narrative and not just a single panel at the end. It was smart to spin the line out of such a big Event, but they should've been involved in the story in some capacity; that would've served as a great way to increase reader interest and introduce us all to the characters.
I dont think the release schedule was a huge problem though. It took a few months for all those books to start, and if DC had waited much longer any hype Metal had managed to bring to the New Age line would've faded completely.
Really, I can't find much that DC did wrong with that line at all. Lots of marketing, strong creators, diverse casts, all spinning out of a big Event.....new characters and initiatives like that are always a risk that'll fail 99.99% of the time but I feel like DC did as good a job with their risk management as they could.