Pruchased Videos are only available for download for a limited amount of time. Somehow this period ranges from about a week up to several months.
But even with the slowest Internet in town that time should easily be enough to download the video and store in an external Hard drive/ USB device or so for eternity.
However I also would be pleased to hear from an admin or Site authoritan why things are settled that way. Why is it even a limited time? I mean, the term "purchase" does indicate a lasting ownership, not a time limited "rent". And on the other hand: Why are the time periods so different from video to video?