When it comes to age, you can't do anything when age is no longer by your side, particularly in football game which involves strength, agility, Mental energy, and lot more to play. Looking at his performance in this world, it's clearly seen that he has nothing much to offer in the field Play, and I don't really think that he suits the kind of striker that Chelsea needs right now.
However, buying him is still not that bad since he's on free transfer and there are other striker already in Chelsea.
You are biased - from your words it seems that the entire World Cup he sits on the bench and does nothing, but in fact he earned a penalty + scored it, plus that goal that was recorded (fairly) for Bruno Fernandes was only possible because of Ronaldo's jump. And in general, during the time that he played, he played in the favor of the team. In the end, it's very strange: Ronaldo is criticized for "doing nothing but scoring goals", but when he does not score goals, he is criticized in the same way.
The same criticism has also been felt by other players before the Ronaldo Era, fans always want something more for players who are considered to have better skills than other players. When the player can't live up to the fans' expectations, the criticism will come at him relentlessly.
Ronaldo can't play as well as he did when he was young, the age factor has made some appearances decline. I admit, the coach didn't include Ronaldo in the main squad when Portugal beat switzerland because his contribution was so lacking. The coach does not want conflict in the team, he prefers other reasons that are realistic than telling the public the truth.