The problem is that people keep saying HEARTHSTONE HEARTHSTONE HEARTHSTONE when Magic isn't comparable enough to Hearthstone for that argument to be meaningful beyond "card game." Like, I legitimately feel some of you guys are just as dazzled by the Hearthstone cash numbers as Hasbro executives are and aren't seeing reality: Magic is an ultra fiddly, timing-specific game that will never, ever, ever appeal to casual game players the way Hearthstone does.
It already does. There's something like an estimated 20mil playing the paper game, with only like 100k active in the competitive scene. The game isn't as complicated if you're not playing in a tournament setting, aspects of the game essentially don't exist until someone goes 'Sylvan Library + Chains of Mephistopheles'. Arena is trying to be simplified magic for the noobs, AND complex magic for the enfranchised AND it's doing it for free. This will be its strength, it's a teaching tool and a retention tool all in one, and it looks like it's doing a good job of it.
It would be exceedingly easy for them to hit multi-million player numbers if they put codes in booster packs, reach is that huge. Now lets ask why they haven't attempted pack codes with MTGO. It's not because of the in game economy, it's because they could not handle the sudden influx of players that would bring (I'm under the impression that shit runs on a single server, which I find hilarious).
Hearthstone type numbers aren't impossible. The common MTG player probably also doesn't see a huge difference between this and Hearthstone. Full-control is there for us, streamlined controls are there for them. I'm actually very impressed with how much they still present with the not-full-controls.
To wit, everyone playing fighting games has their own personal preference of game. There are plenty of games that are conceivably better than Street Fighter. But Street Fighter gets a huge player base because people know Street Fighter, and it is after all still a fighting game, so the pros also wind up playing it. When Arena comes out on all platforms, is everything to everyone, and is free to play, it will get way more players than MTGO almost instantly, and will naturally peel players away from MTGO. Call it audience gravity or something.