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 a bad since he's on free transfer and there are other striker already in Chelsea.