Why wouldn't a client complain about noise? it's degradation of image quality, unlike grain
In this case I'm taking portraits of pupils performing concerts, and I cannot go in front of the audience even to just move around.
It would not be a cost effective to invest in whatever medium format might be suitable, ibis and a long zoom would be needed - I can do what I need to do with 70-200mm @ 2.8, 1/125th, ISO 6400, but as you can imagine to keep ISO at 6400, 1/125th is right on the edge of acceptable shutter speed when using the lens at 200mm.
It's not essential that I upgrade, I just don't like working at the edge of my comfort zone.