I don't know if it is going to be replaced, honestly. They went "L" status on the 70-300, and came out with the 200-400L (at least it's been announced).
If I was on the fence, and needed that long of a lens, I would just find a "cheap" 100-400. Or get the 70-300L
Copied the following from another thread where I posted some info on the 70-300mm f/4-5.6 IS L. Keep in mind many of these will likely apply to the rumored 100-400mm f/4-5.6 IS L, except, hopefully the focus/zoom ring reversal design of the 70-300. The 70-300L has a lot of positives, the section below talks about the negatives. This was originally written in context of a comparison with the 70-200 f/4 + 1.4x:
Negatives on the 70-300mm:
1) slower aperture - yes, if you need the extra stops, this won't be the right lens. However, I've shot this at weddings and receptions with and without flash at ISO's up to 3200 on a 7D without issue. The reach on a 7D in a church or outdoor wedding outweighs the slower aperture when you need focal length.
2) The zoom and manual focus rings are reversed on this lens when compared to the 70-200, the 24-70, the 24-105, 16-35, etc..... In other words, the far ring is the zoom, the near ring is the manual focus. This is frustrating, for one, it breaks what you've learned to do with the zoom ring as you find yourself reaching for the wrong ring at times. Second, it makes you extend your arm further from your body, leading to a more unstable stance when shooting. I like to keep my arm close and tight to solidify my stance. On the 70-300 you have to move your arm further out to handle the zoom ring. It's a PITA, I have no idea why Canon did that. It's the #1 reason I could dump the lens if a new 100-400mm f/4-5.6 IS L were to appear.
3) The hood, filter mount and lens extension....the hood "looks" cheap....no fancy tulip design, it's a straight up barrel. It just looks dumpy...but it works. The filter mount is 67mm which means it does not fit the filters I use with the 70-200mm or several other lenses I have...so if you use polarizers or ND's, just know you might be buying more to fit it. Note the lens is not internal focus and extends 2.11" at 300mm full zoom.
4) Variable aperture - you need to account for that in your exposures, especially when shooting in manual mode and tracking/zooming in-out at the same time. The lens max apertures by natural focal length (unadjusted for crop 1.6x body) are:
Overall, if you want a solid, light weight, and sharp solution, the 70-300mm is a great choice, assuming you can deal with the reversed focus/zoom rings (god I hate that!!!).
I bought it to use on the 5D3 and the 1Dx in order to recover some of the focal length lost on a full frame body, when compared with the reach the 7D had with the 70-200mm. I'm selling my 5D2 and my 7D once I have the 1Dx in and I've become comfortable with the 5D3.
I wanted to recover some reach I'll lose when the 7D is gone, and I didn't want to have to rely on the 1.4x extender on the tele-zooms all the time. I didn't want the current 100-400mm and with no new version of that in sight yet, it came down to either this lens or the 300mm f/4 IS L prime and the 1.4x tele-extender III that I own. The zoom proved to be much more flexible for the same cost and overall sharpness.
If a new 100-400mm f/4-5.6 IS L comes out and does not have this dumb focus/zoom ring reversal, that would be the one reason I would sell this lens, however I would have to believe a new version of the 100-400mm would run about $2600 and not be available until 18 months after announcing it, given Canon's recent trends.