This always happens. Everyone gets attached to a Doctor then he regenerates.
When Tom Baker regenerated Peter Davison had a hard act to follow but his regeneration brought along a whole new vulnerability and youth to the show. I thought Peter played his part very well.
Then the backlash begins. I can remember people saying horrible things about Colin Baker and how "he had ruined Doctor Who".
Watching some Colin Baker episodes the other day,I could not possibly see how he had ruined it. The guy had charm,a ruthless streak and was sometimes a bit of a coward. (although like Patrick Troughton playing the clown, the coward part was just to lure the enemy into a false sense of security.)
When Sylvester McCoy regenerated into Paul McGann critics slated it. That it had become "transatlantic". They also said that the series did not fit into canon as the Terrileptil destroyed the sonic screwdriver in The Vistitation.
I did not like the idea of Kit Eccles playing the part of the Doctor at first but when I saw him on the screen with those cards,I instantly warmed to him. You could see the old Doctors in there shining through to a new modern audience.
I also didnt like the idea of a pop singer playing Rose. Billie Piper proved me wrong. She was absolutely fantastic!
Conclusively this always happens and I hope David Tennant's Doctor will be memorable like all the others.