The problem I see with long-term data storage on any kind of media will be the ability to read that media far into the future...not from decay, but from the simple lack of working drives with which to read them.
M-Discs are SUPPOSEDLY rated to last up to 1000 years in storage (Yes, one thousand). Let's be more realistic though, and just go ahead 100 years. Can you guarantee with absolute certainty that there will still exist functional optical drives in 2118 that can read M-Discs burned in 2018? Or, at the very least, that the Average Joe will have immediate access to the very few that may still exist in museums? The discs themselves may be perfectly fine and all of the data on them 100% intact, but if you have no means to read that data off, they're worthless.
Honestly, the best thing to do every 10 or so years is to refresh your backup/archival media. Keep up with the technology, transfer it to newer storage methods as they come around, rather than stick to the notion that the same data storage technology is gonna exist and always be readily accessible en masse for 100+ years.