I am not familiar with the careers of professional songwriters. But it seems to me that bands or singer/songwriters tend to have much longer success in their careers as performers than they do as recording acts. To me, this would seem to indicate that they can continue to sing or play but are having a harder time composing at the same level. Question is if this is so, and if so, why? I can think of several explanations.
[li]Everyone has only a certain amount of songwriting potential in their mind, and it’s used up early, then it’s gone. (As a practical matter, this may take the form of writing songs that sound like different versions of their earlier songs.)[/li][li]Aging[/li][li]Zeitgeist. Styles change and different flavors come into favor, and the type of songs that were popular when the singer/band hit it big are no longer so.[/li][li]Burnout or lassitude. As some entertainers become more successful, they lose the drive that they had as younger up and comers, and don’t work as hard.[/li][li]Relating to the prior two points, is entertainers who take a break. Having hit songs has a momentum of its own, and creates excitement for the new releases. Taking a break cools this down, and allows the space to be filled up by the next big thing, and makes it harder to reach the same level next time around.[/li][/ol]
What argues against the first two being the sum of it is that in theory these singers or bands could just begin hiring others to write their material. But it could be that having initially made it by doing it on their own, they tend to be reluctant to switch their methodology. A test of these would be whether professional songwriters tend to similarly decline
Regardless of the reasons, it does appear that singers/bands can sometimes stick around as performers in spite of their inability to crank out new hits. Presumably this would be because to do this they only need to appeal to a smaller group of patrons, and they can be carried to some extent by their die-hard or nostalgia-driven fans, but they will have a hard time making new ones.