Hearthstone is very well designed for what it is--an online-only Magic the Gathering that is accessible in a casual way.

I simply won't spend on Magic Online since I can't exit. In addition, why should I spend hundreds of dollars on something whose value can plunge drastically?

If you have actual physical cards, you can sell them. Powerful modern cards hold their value reasonably well.

You can exit from MTGO pretty easily? There are automated bots that will buy your cards for tickets, and those same companies will give you cash for those tickets (95 cents per ticket right now, not a terrible rate).