I think the main reason why they (still) are the World's Greatest Rock N'Roll Band is the comparison with other live acts nowadays. There are not many people in Rock music who can keep up with them and their popularity. The Stones are the best act around, in my opinion.
But compared to themselves in former years, they are musically a mere shadow of what they once have been. I guess we knew they won't grow old gracefully anyway :-) And I still get my kicks of attending a stadium show, hearing 50.000 people singing along, screaming and shouting.
They are way past their prime, muscially speaking, but still the best act around!
And I think they are still able to record great album!