I think that no matter how you backup, the backup needs to be checked and refreshed every so often.
I don't think that Cd's would last 50 years without some deterioration of the disk.
Though I am old enough to remember the TV program 'Tomorrows World' coating then with jam etc and saying that the disks were virtually indestructible. Not so.
There are a lot of things that can go wrong what ever way you backup. I advise, backup to two different places and then keeping the two as separate as possible. But whatever you choose, check every now and then to make sure that something damaging has not occurred.
I use an external harddrive and 'Dropbox'.