I personally think this is a win-win for canon. Each of these lenses have been upgraded to some extent, and by continuing to support the old and completely viable older glass, new lens purchasers are ensured they will able to keep their top-end glass for 40 years to come.Doesn't that somewhat create Apple's iPad problem though? In that a used or refurbished one is a better purchase than a new "low-end" model? At retail, is the EF lens, even at lower prices, compelling compared to the RF lens?
I would imagine most people looking at the value side are considering used EF lenses. If I were looking at retail price, between the RF 100-500 or EF 100-400 II, the RF looks pretty compelling. But if I'm value conscious, why wouldn't I take a used 100-400 for $1400? Or for the 16-35 vs 14-35 question: It's $1099 vs $1699 new, or $600 used.
That "small" delta on the new RF lens means no adapter, smaller system, and perhaps better performance vis a vis the IBIS / IS cooperation. It's also newer and (usually) lighter.
And those conundrums are at full-retail for the RF lenses. Once we start seeing discounts on RF, the deltas will make new EF even less desirable.
Lens Model EF New RF New EF Used RF Weight Advantage 100-400 / 100-500 $2,399 $2,799 $1,400 225 grams / 7.94oz 16-35 / 14-35 $1,099 $1,699 $600 75 grams / 2.65oz 24-105 $1,099 $1,099 $675 95 grams / 3.35oz