I'm sorry to hear that, but he's just following the precedent of many former actors who played the role, including the last two. I adore Capaldi, so I hoped he could one-up his predecessors by doing one more series, but I suppose it just wasn't to be.
Unfortunately, my cable service dropped DW right before Series 9, and I haven't got around to watching that last season which aired, what? A year and a half ago? I need to get on that.