I think they should cancel the book.
I also have to second this. ASTONISHING X-MEN was launched for the sole purpose of having Whedon and Cassaday doing an X-Men book. It's that creative team, not the title, that's made it the #1 selling X-book (at least on the 4-7 months when issues actually ship per year). There are already a large number of X-books on the racks and it's not 1995 anymore, where any X-title could crack the Top 20 by existance alone. Hell, as recently as 2001, BROTHERHOOD #1 was a Top 10 book. Nowadays, solos starring Gambit & Rogue can't last beyond 2 years. Of course, hardly any ongoing in ages has lasted beyond two years, with exceptions being NEW AVENGERS and CABLE & DEADPOOL, amung others.
We have X-MEN and UNCANNY X-MEN and without that A-list team I really see no point to a 3rd ongoing X-Men title, considering we also have NEW X-MEN, WOLVERINE, WOLVERINE: ORIGINS, EXILES (to some degree), CABLE & DEADPOOL, and inevitable X-mini's that come out in buckets every year.
Millar & Hitch could maintain sales, but I doubt they could even ship 4 issues in a year, which was the lower of the Whedon/Cassaday numbers. Besides, he no doubt would Ultimatize it, because that is all he is capable of doing in 616. On the plus side, his Cyclops would be an unquestionable bad-arse, least going from how he wrote Ultimate Cyclops. But Hitch's been unable to get in even 4 issues in a year in ages. But I could see them launch a seperate mini vs. continuing ASTONISHING for another 6-12 issues for that.
In a way, ASTONISHING without Whedon & Cassaday is like NEW EXCALIBER without Claremont; NE was launched pretty much as the obligatory "Claremont X-book" because apparently there still is a market for that, but when he fell ill, the book was stuck in limbo for a lot of 2006. Marvel's answer there was to do fill in stories (same with EXILES, which Claremont was supposed to take over for in 2006 and will not get to in 2007). Of course, EXILES is a long-running franchise title that has had several writers before so it's not the same; NEW EXCALIBER is as close to an ASTONISHING as one can get; it's purpose is it's writer, not the franchise itself really. Marvel chose to get on a filler writer who did well enough, but with ASTONISHING X-MEN I am not sure there is a real purpose beyond Whedon & Cassaday, aside for sales. It operates mostly free of whatever continuity is happening with the X-Men and other books barely acknowledge it (aside for a revived Colossus), perhaps because of it's infrequent shipping schedule (neither a monthly or a bi-monthly or even a quarterly, it's as erractic as hell.
I wouldn't be surprised if they kept the title alive and got a new creative team for it. Lord knows Marvel will have 'til the summer at the earliest to decide, and that's a while from now.