Hardly a shock, though the shift to another night NEVER bodes well ("we moved it and fewer people watched" - well, duh: shows on the slide are never HELPED by a shift in the schedules... name me one show that was already down from its peak that was actually raised by a schedule shift? None? Well, none I can think of).
The problems were always going to be thus: the series started already into the term of Bartlet so already into a finite 7 seasons (because those sort of major transitions are never easy to handle: it would have been like trying to keep Buffy going without Buffy as the slayer. As much as some wanted that as the continuation, I was never for it). Additionally, for all that I believe season 6 was something of a return to form, the post-Sorkin era was never going to hit those high spots. Season 1 was awesome, season 2 a brilliant follow-up, season 3 was moderate - 9/11 had a big impact but the problems were there before that, season 4 had some excellent moments, and season 5 didn't capitalise enough on the legacy Sorkin left, though again there were some classy moments. Season 6, by all accounts - I still haven't borrowed it from my friend yet - was something, but not everything, like a return to the better times. Season 7, I have no idea about (please don't spoiler me too much!), but - and this is my final point - once John Spencer died it was always going to be a struggle to justify its continuation. Two key characters lost by the end of season 7? How could there be an 8th?
Moving the night was never going to help, but sadly, the dream was truely over.
It was a great ride and I can't say it wasn't worth taking. Shame to see it go, but it was pretty inevitable.
Postscript: Rob Buckley has an excellent post on this topic that is far more eloquent than mine.