ummmm - // Pink Floyd wrote most of their songs off their trolley! Same with the Rolling Stones and dozens of other bands.
As long as they can perform on stage that's all that matters. //
To reach their level of success, they have already proved that they can create lasting music, and perform it properly, personal indulgences notwithstanding.
In their day, if you couldn't do that, you vanished before you were even heard of.
Sadly, with the advent of 'celebrity' - people are catapulted into the public eye before they have had a chance to find out for themselves if they can hack it, never mind convince anyone else.
That's how someone like this, who clearly cannot handle the 'celebrity' thrust on him, has his failure writ large, and that will be the epitaph to his 'career', ended before it began.