Yes, Ronaldo is already declining. I have seen him miss chances the last couple of years that he would 100% bury before. He did strike a good form at the end of the year though. But can he do it at United with service (or lack of service) from Valencia, Lingard and Martial?
And, yes, he will win the player of the year this year. But you do realize it gets much harder to do it each year. The decline at the very top level is not linear, because a quarter second makes all the difference between top 3 in the world and top 100.