It’s flat-out idiotic of you to accuse singers of being poor musicians for not being able to sing in any key that’s convenient for you, since you’re too poor a musician to be able to change key comfortably on your instrument. You are supposed to be able to play in any key. Singers can’t perform in any key. For someone whose voice sits in the B-C-D area, singing in G will damage them. You can buy new strings or a whole new instrument. They can’t.
In nearly all cases of sung music, the singer sets the key for their convenience and comfort, and if you don’t like it, tough. Either stop accompanying singers, buy another fiddle and tune it a half-step flat, or learn how to play the damned thing.
Sit it out, or suck it up.
I’m not even a singer and this irritates me. Maybe it’s just another instance of violinists getting on my nerves because they always assume that no one else on stage matters. Maybe as a pianist I’m like the cat who walks by itself, and all key signatures are alike to me. But when a singer is on stage, they drive the bus, people.