The next team may indeed be weaker, but I wouldn't give away the title in the whole tournament in advance to Barcelona. There will definitely be teams on the road that will give it a very strong fight, it makes sense, I'm only sure they'll make it to 1/4 for now, that's all.
Barcelona struggled a lot in the Champions League but they seem to have things in order now and with their form they are certainly a contender to win this tournament. Then it would still be a successful season. Galatasaray is normally way too weak for Barcelona, but I've read that playing in front of that crowd in Turkey can be a bit of a nightmare. I don't know who will play at home first. if Galatasaray At home gets a good result, they still have a chance.