I second this! I do have the RF 35mm 1.8 and love it, but all the lenses I have EF including the 500mm F4 IS L II works without any problem in the EOS R. So I do believe that with the R5 would work perfectly! You don´t need the new RF lenses. But of course, with the RF lenses you have the new control rings and probably s slight gain in image quality due to the short distance to the sensor. So you can do a soft transition to RF, buying lets say...1 lens a year and gradually replacing the EF lenses to RF. I am looking forward to use the R5 with the 500mm!That's ridiculous, the lenses I have work fine.
Saying that...Canon Come on!!! There is no photokina just announce the camera and release it ASAP!!!