I agree his average has dropped slightly, but that does not undermine the fact that he still his one of the greatest batsmen of all time. Sachin Tendulkar was scoring 100's from the age of 17 against some of the best bowling attacks in the world on some of the most seamer friendly pitches in the world.
When he burst into Test cricket he was playing in an era that had seen some of the best bowlers emerge. These include, Courtney Walsh, Curtley Ambrose, Wasim Akram, Waqar Younis, Shane Warne, Mularitharan, Allan Donald along with Brett Lee & Shoaib Akthar at their peak towards the end of the 1990's.
Secondly, Tendulkar has scored only 19 of his 51 Test centuries in India, although admittedly there would be a few more scored on the flatter pitches in the sub-continent. It should also be mentioned that he has been dismissed 20 times between the score of 90 and 99 and 17 times between the score of 80 and 89. If those scores had been converted to hundreds, we would have seen more centuries than he has been credited for.
I will admit that Alastair Cook handles captaincy much better than Sachin Tendulkar did many years ago. Cook has not only accepted the challenges of being a captain but has flourished from it given his recent form.
In my personal opinion I believe that Sachin Tendulkar is a better batsmen than Alastair Cook. I also stand by my opinion that Sachin is one of the greatest batsmen of all time, not the actual greatest but among some of the greatest such as Don Bradman, Garfield Sobers, Viv Richards, Brian Lara, Jacques Kallis and Graham Gooch.