I think Carra is being smart, and he knows his body better than anyone and it’s time to hang them up before someone tells him to. There is no doubt we’ve seen the Indian summer of his career this season, which I very happy about, not just because it’s good for us, but because it allows him to go out on a high (even if it does leave us craving a little more), but the fact he's really just playing one match a week, I think he knows if he signed up for a another full season he might not look so good if we expect the same of him in each and every match next season.
Your right that integrating new players could be an issue, more so if we need to bring in two for the same position. That is one reason why bringing in a more established player may make more sense, especially someone familiar with the prem and or the system we play (yes that maybe a weak endorsement of Williams rather than a hotshot from abroad). But I'm also interested in the reaction we get from Skrtel (assuming he stays which I hope he does) to some competition coming in. I'm hoping we'll see the similar reaction we got from Jose and Downing, and while we can't change either Agger or Skrtel into a loud mouthed scouser, we might start seeing the best out of them with some new blood nipping at their heels.