Because the media is expensive and not as capacious as a hard drive, you'll have to choose what's really important and perhaps divvy it up across discs. You may view this as an opportunity to clean house or a deal-buster.
Also, as always, optical is relatively slow: M-Disc BD discs write at a rather pokey 4X/18MBps (6X/27MBps is the BD max), and M-Disc DVD is also 4X, or 5.28MBps. That's way off the DVD maximum, which is 16X or 21MBps. But it's a once-in-a-while deal, so just start your backup, minimize it, and go on about your business.
Why not online archiving?
Online archiving is certainly an option, but even in the age of ubiquitous broadband, online storage is relatively slow, even slower than optical in many cases. And relatively expensive. And unavailable when communications systems are down. You don't know who has access to the data, and you don't know how well the data center is backed up.
Yes, I have a streak of paranoia, but it's born of experience. There's nothing quite like knowing there's a backup in your safe deposit box or at your relatives' house. Not that you shouldn't store a copy online as well.
I'd strayed from optical for all the usual reasons: lack of speed and capacity, expense, bad discs, etc. But I'm back and fully intend to keep my most important data, the stuff that can't be replaced, archived on M-Disc. BD-R HTL would do, but just in case I do live a thousand years, I'll use M-Disc.
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