I think Sony has been holding back the A7SIII waiting to see what Canon releases. I think if not for this R5, we would be seeing an incremental update from them.
Sony will have to do even better than the R5 spec wise to stop people from jumping ship and coming back to Sony as if its even level it won't stop a mass exodus.
I'm a current Sony user and have been for the last few years, but for 9 years prior to that was a Canon faithful. The only thing that can stop me from going back to Sony is their body prices. I don't NEED the R5 but i'm looking to buy one, the R6 may well suit my needs better as long as the joystick, dual card slots, EVF and battery remain, and If even the R6 price is ridiculous then I'll stick with my Sony A7III and add an A7RIV. Exciting times ahead