CBRs own articles seem to confirm this -- that digital sales are additive to print comics and doesn't cannibalize business. Though it's usually the suits at DC/Marvel that are saying this, so I take this with a grain of salt. After all, if digital were cannibalizing print sales I somehow doubt they would openly admit it, since they seem to be making a big effort to have LCS owners embrace digital as something that can benefit them as well. But just from reading various threads on CBR's forums, there seems to be quite a few comic readers to have left print comics and jumped to digital.One of my LCS owners claims digital sales aren't anywhere near what publishers had hoped they'd be and that print is still dominating by a long shot. I can't verify this, but it's what he says.
But going back to my original point, I don't think it's an impossibility that comics could be a primarily digital market, with a small subset of buyers still buying print. I don't think it will happen anytime soon, but perhaps in another 20 years or so, why not? If it did happen, I would see it as being part of a larger wave of publishing in general moving to a primarily digital format, and periodical publishing in particular. Comics might be one of the last holdouts, since there is more of a collector mentality associated with comics, versus newspapers and magazines.