The singer/songwriter model is the wrong model. Sure, it can work, but you don't often find both skills at the highest level in the same person.
Motown was great because the performers were separated from the writers.
Beatles??
I’ve always been rather partial to bands with an excellent writer who could write to and work in harmony (pun?) with the strengths of an excellent singer, whose abilities said writer probably understands more rationally than the singer him or herself.