If we take only marketing goals, then it does not matter how Ronaldo plays. He remains one of the most popular athletes worldwide. Chelsea will buy him for free, which may not be such a stupid decision even if Ronaldo keeps his huge salary. And I don't think that Ronaldo is already that bad (more precisely, he became bad in just a year) - last year he was the top 3 scorer in the Premier League and this year he is retired? Hard to believe.
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.
However, the main problem for Ronaldo is that he is too ambitious to achieve the highest achievement and with such great ambition, Ronaldo will never be able to accept if he is too often on the bench. Even though each coach also has his own strategy which of course will make some players unable to play in all the matches, when they don't match the strategy used at that time.
Also, if Ronaldo still really has good gameplay then of course Ronaldo will continue to be played as a starter, no matter how old he is because if Ronaldo fits the strategy implemented then of course the coach will also continue to play him.
But what has happened lately, even Ronaldo does not always play the full 90 minutes or play from the bench, when compared to Messi the situation is very different because Messi can still be relied on by the team in every match.
Therefore, if there are no more big teams who are going to sign Ronaldo, then it would be better for Ronaldo to retire, because it will only be useless if Ronaldo continues to force it but in the end Ronaldo will only be on the bench more often.