You might find a 30 day free trial has more sign-ups and conversions, but could entail more support costs ("I forgot to cancel") than the 30-day money back guarantee.
I admire these companies. They know they have excellent products and the user can get back to them anytime they want. I support them and give them positive reviews. But they are rare - most use the old bag of trick to "increase conversions" whereas they just cause a lot of frustration (and increased support cost).
But you're right that a free trial would produce more sign-ups and conversions, potentially enough to offset the additional support costs once we're in a better position to handle them (or once the product is better at preventing the need for them). Definitely something to think about in the future.