Bear in mind that he's scored 44 this season.
(a) That's in a Real Madrid team that is better than every team in La Liga except Barcelona, and can basically expect to dominate virtually every game. For them to only get 76 points has been a truly dreadful performance. That wouldn't apply at United, who face at least five teams that are roughly as good as them, and arguably they don't even dominate against the lesser clubs, particularly with Mourinho's cautious approach.
(b) Real reached the Champions League final. Again. It's highly unlikely that United would do this, so he would have less games to play in.
(c) Even in a terrible season, Real have scored 94 league goals and 30 Champions League goals. United scored 68 in the league and 13 in the Champions League. So Real scored literally 50% more in the two major competitions (53% to be precise). Great player though Ronaldo undoubtedly is, he is largely reliant on others to make goals for him, and he will get way, way less chances at United than Real Madrid.
(d) While he certainly doesn't look finished yet, that day will come. We don't know when this will be. One day Messi won't be able to do it any more. This is inevitable. Sooner or later, Ronaldo will stop scoring, or he'll retire at the top. Either way, he's not guaranteed to keep churning out prolific seasons indefinitely, in fact the opposite is guaranteed. This will end, and it will end soon.