"why do you think Carragher is regarded as one of the best defenders in the country?"
I've already said that I'm not aware of this. I don't agree that he is one of the best defenders in the country. If it was the prevailing view then my answer would be: "Carragher is regarded as one of the best defenders in the country because most people are clueless", or something similar.
"....and is a mainstay in our team?".
As I've already said, we play in an ultra defensive formation, and he does a good job of shot-blocking in and around the penalty area, and general mopping up. Yet, how many credible title challanges has this (including the Houllier years at full back) yielded? Plus, he can always go running to his media stooge to have a sly moan if things are not going his way and get the carrapologists fired up.
"If Jamie Carragher is not particularly strong, poor in the air and slow, then why has he been our first choice CB for six years now? Why have there been no attempts to drop and replace him? Why do you think he gets his game?"
Again, suited to the ultra-defensive formation. Rafa tried to ease him out the frame into full back - remember Bascombe doing the hatchet job on Skrtl, histrionics on the pitch? Also, playing alongside a dominant player such as Sami Hypia masked Carragher's shortcomings to some extent.
At the start of the season we played a more attacking game and the defence, with Carragher in particular, were all over the place. We were shipping far too many goals, Rafa bottled it and went back to the pack-the-defence method - happy days for Carragher.