Originally posted by Class A But it isn't.
At most it is an alternative, but a number of reasons have already been mentioned via IBIS has many welcome advantages when used in DSLRs.
IBIS is a valid (and, AFAIC, better) choice for DSLRs just as it is a valid (and, AFAIC, better) choice for mirrorless cameras. Those who don't mind the image degradation that is part and parcel of moving lens elements around in a lens, may combine IBIS with lens-based stabilisation, but there is no doubt that even IBIS on its own is a great choice (including for DSLRs).
I had a look at a friend's (yes, I do have a few) Nikon a few years ago that had a VR 80-200/2.8, or whatever Nikon has in that range, mounted to it.
I did like how the viewfinder image was stabilized but I also saw how it could make a photographer very sloppy very quickly. One of the side advantages of IBIS is it tends to keep the photographer from using sloppy handholding techniques.
Whe ILIS is definitely superior for longer lenses I do question exactly how many users it will benefit both because it is a fairly small user base using 400mm+ lenses and because those lenses tend to be heavy enough to demand tripod use anyway.
I can kind of sort of hand hold my A600/5.6, but there isn't any chance I could do the same with a 600/4.
Otoh, I like that every single lens that I own has the advantage of image stabilization even if that advantage is somewhat lessened the longer the focal length and I like that there are no optical compromises made in my lenses to accommodate floating elements.