Hi, i have just unlocked this today, not sure if there has been an update or not to the above solution, but you only need 6500 fame to unlock the better ending. You will need to play on a decent level and try out the bonus challenges it throws at you. My first try was on easy, and only ended up with 3500 fame after the 4 concerts.
On my 2nd try, I played on expert vocals, 5 starred most songs with a few golds thrown in and ended up with 8500 fame after the final concert, unlocking some bonus shop items, venues and the achievement.
Notes from comments,
Recommend playing on hard or expert.
If it doesn't pop, it may pop on the start of a new Rockudtama