Best way to backup a 5 minute movie for 50 years?

The best way is to become famous - then let museums and other cultural institutions store it for you.

The next best way is to realise that things like wedding videos get watched maybe twice - and your descendants likely won't care.

You could save the video in a variety of open codecs on multiple media and hope that at least one of them is compatible with machines in 10 years time (good luck finding a contemporary machine which reads minidiscs!)

Ideally, you would set up a legal trust which is tasked with shifting the media onto new formats every few years. That's complex and costly. And, even if you do find a law firm which isn't going to go bust, get bought out, or simply forget - you don't have any standing to sue. Because you'll be dead.

Finally, and this is similar to the first suggestion, you could create a tradition in your family to watch the clip every Xmas / solstice / anniversary. Keep that tradition alive for the next 50 years and I dare say it will become part of your family legend - similar to https://www.theguardian.com/environment/shortcuts/2017/jan/0...

16mm cinema film is extremely reliable. Especially b/w. And it is not rare technology and will be easy to read by future devices.

I think writable blu-ray discs are pretty good but writable DVD are usually not so good in terms of longevity.

Write it to stable film stock. Seriously.