Politely disagree. M-disc for BD-Rs are still absolutely worth the money if you want to properly archive something. NIST has agreed that the archival lifetime of a M-Disc BD-R is 100+ years.
You have to be careful with normal BD-Rs because there are two different types of recording material on the market: High to Low and Low to High (LTH). You want to stay away from BD-R LTH discs as their longevity isn’t as good as the High to Low discs.