I believe that the 24-105L will remain the 'main' kit lens for the next 5D Kits lens (and I voted as such). My take on it, is that 'kit lenses' (even for the 5D level) are 'historically' a more all round lens - and perhaps a bit slower than a 'faster' lens (that's why I see the 24-70 mk II f2.8 as a more 'specialist' lens).
The 24-105L is a well rounded, great lens - and can be used by many FF users to capture quite a variety of beautiful photos, from landscape, to portraits, and various photo genres in between. Ok the 24-105L is not a 'dedicated lens' for those applications, but it can do a number of things 'quite well' - which is why I believe it's a good 'kit lens'.
Of course, some resellers might 'bundle' other lenses, including the 24-70 (mk I old stock or new mk II) with the next 5D. And others might 'bundle' / reduce the price of a prime with it.
I don't have a FF camera, but I had 2 x APS-C Canon camera bodies, including the 7D. I think the 15-85mm should be the 'kit lens' for that camera, but most times it wasn't bundled as such. I use the 15-85mm as my general 'walk around' lens, and it's focal length and usability (IS, USM, etc) work well - though when I need a specialist application, I'll use one of my other lenses, eg UWA, 100mm USM macro, 70-300mm L, etc.
The 24-105L is the 'most similar' to the 15-85mm for full frame, and has a very good focal range. I'm interested to read more people's thoughts / takes / perspectives. I have used a 5D, and know that many FF users find the 24-105L their 'go to lens' for many situations.
Cheers, and regards all!