The 100-500 should be a good substitute for the 70-300, and both the current 16-35 and 24-70 f/4 have not suddenly stopped working. I want these lenses myself, but there are really only so many that can pop out at once.
We are missing the super tele's, MP-E, macro's, tilt shifts, cheep primes, and the f/4's. Canny expect Canon to have replacements for the whole collection in just a couple years.
Personally I am looking forward to the RF 16-35 f/4, 24-70 f/4, 24 tilt shift, 135 tilt shift, and 200-400 f/4