The 24-70 f/4 IS is a case in point. My guess is that in time, this will replace the 24-105 but it will be a gradual transition. If they launched the 24-70 at the same price as the 24-105 then they would never sell whatever inventory is in the channel and they would devalue the 24-105. So instead, they let both lenses co-exist for a few years, gradually reducing the price of the newer one. The 24-70 started out at £1500 in the UK. It's down to £935.
I suspect the same will happen with the 16-35 f/4 IS. It will be expensive to start with but reduce towards the 17-40 over time.
The reason the lens drop do much in value is because they are over valued out of the gate and nobody buys then. Canon lately has been pricing their new lenses extremely expensive and Sigma has been coming in with great gear at a reasonable price, forcing Canon to lower their prices. Ain't competition great.