I don't think Ronaldo is what he was. He still scores a lot of goals, yes, but many of these are penalties. I don't think he really improved Juventus when he was there, and yes he is getting older and towards the end of his career. He is still a good player, and will add something, but it's not enough to turn Man United into title winners.
I think a lot will be made of the fact that he is returning to Man United, but I don't think he has any special feelings or loyalties there... because a few days ago he was genuinely considering joining Man City, their biggest rivals.
I do not think United needed a lot to be title contenders, they were already title contenders in my point of view and they still are, not saying that they will definitely win it or they are the biggest contender out there, but being in the top three teams of a league usually means you are a contender.
With Liverpool having one big injury, or city having one, that is going to make United a bigger and possible championship contender (not championship league, champions of the premier league), so it is really a great transfer because they were already good, and they added someone to make it even better. Plus, I do not think that Ronaldo is washed as you suggest, dude is still Ronaldo and he can bring so much to the team, he is great on and off the floor as well, he could be both great while playing, but he could also be team leader to help the younger people on the team as well.