I am keeping all of my EF lenses. The only RF lens I have is the 100-500mm, and will be adding one RF 1.4x TC. Those who already have the latest version of the EF 100-400mm aren't gotting rid of it because oil its great IQ. The same for the very expensive EF Primes from 400mm to 800.
I have a bunch of EF lenses as well, works just as great with the adapter on my EOS R5 and R6.
But, I'm trigger happy on the RF 400mm f/2.8 (and perhaps sell the EOS R6, RF 100-500, EF 16-35 f/2.8 III, EF 24-70 f/2.8 II and my EF 70-200 f/2.8 III to partially financing it)
I'd like to isolate my subjects more. I know the 100-500 is versatile and light, great for hiking and in my backpack and the 400 is rather heavy and not that versatile at all. I guess the bad comes with the good. I'm using my 100-500 exclusively at 500mm (sometimes at 400mm) and the 400mm f/2.8 with a RF 1.4 extender I get that extra reach.