He will certainly score a lot of goals, and he will certainly sell a lot of shirts, and he is certainly - at whatever age - a statement signing. I just don't think he's the answer to their problems, and is not the type of player to turn them into a team that will challenge for the league title.
Ronaldo can provide goals and income from shirt selling for Manchester United but he can not bring the club to Premier League championship. If he has such power, he would help Juventus win Scudetto in the past season.
I certainly not saying he's a bad player, he's an all-time great and his record speaks for itself. It's just maybe not the best way that they could have spent the money.
He is one of two greatest players in last 20 years and one of greatest players in history but in modern football, it is not easy for a single player to help the club win a title. Football nowadays is very competitive and even super stars need to have support from others. Basically football is a sport of team, not of a single player.