Since this is the role of a lifetime, and since he has accomplished so much in his life and his career, there's every chance Capaldi could stay in the role longer than Tom Baker. Think of it, we could have the same Doctor for longer than three to four years (as has always been custom outside of Baker). Ten years from now, he could finally decide to hang up his own Sonic Screwdriver and the difficult search would begin again. He has the power to do it - he played Malcolm Tucker for around seven years, so why couldn't he do that for The Doctor? Why couldn't he do it longer? Yes, the show is all about change, but this is going to be a different show with Capaldi in the driver's seat.