If XenForo switched to the PayPal REST API not only would it allow users to cancel subscriptions, it allows for some other useful things. For example you can have subscriptions that start with an upfront cost (or nothing upfront) with the subscription starting on a future date. One place this would be useful in stock XenForo is to allow users to change their subscription length. Rather than have to wait for a monthly subscription to end before resubscribing to a user upgrade, you could simply start a subscription now that starts on a future date. I've been playing with this feature for a project. For example this has a $399 up-front cost, and then an annual subscription that doesn't start until 2 years later.
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You have
so much more flexibility with the PayPal's REST APIs.