I think Ringo will be the Last Beatle Standing because Macca simply has too much stress now in his old age, due to his one-legged wife. So far as I know, Ringo lives in peace, does what he wants, has a leisurely lifestyle. Paul is still working very hard with his music, is still trying to win back his catalog from whoever bought it from him this time (first it was Jacko, now its whoever), does not live leisurely because of all the pressure.
Paul will definitely bite it sooner than Ringo.