If the download is freely available from that website (which it is) and you aren't required to enter any authorization details to download it (and I wasn't), then I don't see how they can stop you downloading it beyond 3 years. It's copied onto an SD card. The Nav unit (Discover media) just reads this card, I don't believe it does any validation checks on it.
Besides, there are always ways around it even if they do. I had an RNS 510 in my MK6 and the map updates were easily attainable if you looked hard enough.