Agreed completely, I absolutely love that button and miss it when I'm using an EOS R. I have mine set up to switch focus modes while it's held down, I always have my camera in Servo focus, and when I need one-shot I just hold down the DOF button and it will switch to one-shot while it's held down for a few images, but will let me immediately be back in Servo the second action starts back up.
My 1DX2 has two of these buttons, and I have the secondary DOF button set to playback, so that when I'm shooting with a 400mm f/2.8 or etc I can hit the playback button with my right hand on the grip and my left hand still holding up the lens.