Having 70 million people paying $6 a year on average is better than 300,000 people paying $1000 a year on average (and MTGO is not anywhere remotely close to that -- Magic's revenue as a whole is only about $300m and MTGO is a small fraction of that.)
Magic Duels -- a mediocre offering with no streaming community that's already been EOLed -- has 2.7m accounts on Steam alone with 70,000 people still playing weekly. Arena is
far more appealing than that and is gonna have a full-court-press on Twitch to get people interested. I'd be shocked if they get less than 10m signups in the first year, and it's trivial at that scale to make more money than MTGO even with small spenders.
If you want another indicator of how much upside there is in this market, Konami's latest Yu-Gi-Oh app has 50m downloads in a year:
https://www.konami.com/games/corporate/en/news/release/20170823/?cm_sp=01-_-release-_-20170823-e