PsychoTherapist is correct. I remember back in 07/08 ITV's intentions slipped out and were quite clear that once the then current contract expired in 2010, they would not be renewing it. And, well, that's exactly what happened.
It's a shame that they could never quite return to the essence of what made the programme so great originally. Even though the 'Young years' had a good run, I'd often find some of the writing rather unimaginative and the newer characters a bit cardboard. Perhaps if they had reduced it to 1 hourly episode a week, or as others have suggested, back to the 30 min format, its lifespan would have increased a bit more.
As for the last ditch attempt at the revamp, I felt the stylistic change wasn't necessary. I could see what they were trying to do in reinventing the show by changing it's overall identity by dropping the theme tune and giving it the CSI look, as well as an abysmal score. I have to admit though, in the latter half of that series (2010 onwards), some of the writing did become better and the stories I found quite gripping - even though the uniform cast had been reduced to background extras for the most part.
I think the show did well to go beyond its intended lifespan, and I truly believe if it had been given a bit more TLC, it would still be going strong today. After all, Casualty is still limping along over on BBC 1...