How good should Adele's guitarist be?

If an act is a vocal act, the guitar and all of the other instruments are there to complement the singer, not distract from her.

Same with the focus for a product.  The main feature needs to differentiate the product.  The rest of the features are supporting acts that must not overshadow the main feature.

But if the guitarist was better, maybe you could get guitar fans to come to Adele concerts. But you would start to lose the vocal fans who don't want to sit through long guitar solos.  The guitar fans can go see Kenny Wayne Sheppard.

Adele has a good guitarist, but he knows how to support and complement the vocals. KWS has a good lead singer, but he does not overshadow the guitar.