As I said, I have - and love - a Sony a7R V with the FE 24-70mm GM lens, but that is a large and fairly heavy camera, so for me, the purpose of getting an a7RC is to replace my RX100 VI with a better but still very compact camera to use on occasions when size matters. I considered the Sigma 28-70 DG DN, but it is 4" long and weighs a pound compared with Sony FE 28-60 f/4-5.6 which is under 2" in length and weighs only 6 ozs. The Tamron 20-40mm f/2.8 is almost twice as long (3.5") and heavy (13 ozs.) as the Sony 28-60mm f/4-5.6, though it is a faster lens.
I do think Sony ought to release an improved version of the FE 28-60: a compact, light weight, mid-range zoom lens, to complement the a7RC and make it a viable alternative to the Leica Q3 for people who want a high-quality compact camera but would prefer to remain within the Sony ecosystem.