I'm a physics layman, and I'm having some trouble with uniting the content of your comment with the fact that existing magnetic confinement experiments have reported maintaining a plasma at the right temperature for longer times (not with fusion, but with microwave heating, and with the power of those heaters in the 10MW range).
Have I understood the consequences of those reports wrong? Does the heat loss you talk about only occur with fusion? (And if so, is it even a problem if the conditions for fusion to occur can be created by external heating this "easily"?)