The whole online vs offline play has been going on for years now. Only SDS has the answer since they have the metrics, but I would be curious as to where the majority of their revenue stream is coming from.
Casual players and "hardcore offline" players (I place myself in the latter) prefer to play offline because, in general, we are looking to have an enjoyable time playing the game our way, on our difficulty setting without having to worry about possible cheaters, freeze-offs, connection issues and the general "toxicity" that seems to come out of online play.
Now before you ask, as a matter of full disclosure, while I love offline and would play it almost exclusively, I am not averse to the online portion, but I play it at the strictist minimum and not a single game more than I need to in order to obtain what I want.
While I think many in the online community are decent enough people and want to play against another person as a way to test their skill, there are those who want to abuse the mechanics as much as possible, and generally make the game as unenjoyable as possible for their opponent because their only goal is to win at all costs. Now I know I am generalizing here, but that crowd is usually the younger crowd, the less "mature" crowd.
The best comparision I can make is a PvE vs PvP environment. Most people prefer a PvE environment because they just don't like to have their gameplay experience ruined by someone else who is intent on having fun at someone elses expense. An old game I used to play, Ultima Online, at one point decided to "split" the world into a mirror image of itself, Felucca and Trammel, with Trammel being the PvE only world. Overnight, Felucca became a barren wasteland because people prefered to play without PvP and when they wanted some PvP would simply go to the Felucca shard to get their fix.
This game is no different. I think both communities can co-exist, and I think SDS has an obligation to ensure that both can get what they want out of this game. There needs to be an offline RS mode (I made a post about that several weeks back) for the offline community so that vs CPU has more meaning. Those that refuse to play online except say events and the occasional BR won't invest money into the game to play a mode they refuse to, but I suspect that many of them would invest a few $$$ more to get some better cards in order to play an offline mode that gives them a REASON to play and a goal to work towards.