You're not trying to get people to forget about the service and pay for one month then cancel. That's not a long-term success for the business. You're trying to extract a commitment to pay money because extracting a commitment to pay money automatically increases their perceived value of your service, their engagement with it, and their likelihood of paying for it.
Forcing the decision-point until when the trial is over will cause them to discount the value of their software because a) they've already consumed it or b) they haven't had time to look at it yet but have had time to totally forget your sales copy about it. You want the big decision point to come at their moment of maximum perceived desire for the software: when they've just come to your website with a headache and you've just told them you're selling aspirin, the magic cure for headaches. At that moment they burn with desire for aspirin, so get agreement in principle that if the aspirin cures their headache you get paid, rather than saying "Take this aspirin and then let's talk about how much you value not having headaches four weeks from now."
After you get people to make the decision, all of their future actions will tend to reinforce that they previously made the correct decision. (People hacking 101.)