Politely agree to disagree and I'll elaborate. Thanks for your input.

LTH are all marked as such. MABL normal (non LTH) discs such as verbatim sells for less than half the cost of M-Discs have the same physical properties as M-Discs, the protective layers are the same, the recording methods are the same using the same materials. Therefore the longevity is the same or near the same without getting into M-Disc's ridiculous marketing claims of 1000 years (when NIST and others agree the poly-acrylic protective layer would degrade and decompose after a century or two at most even in ideal circumstances).

/r/Datahoarder has had this argument several times and the consensus so far seems to comes out to the fact that M-Discs were a DVD-era innovation that in the BD era offer no meaningful advantages in technologies.

I'd rather have two BD's from a reputable company like Verbatim (not fly by night plain white discount bulk BD's from who knows where) from separate batches bought 6 months apart stored properly than rely on one overly expensive M-Disc that isn't going to last any longer and probably isn't made to meaningfully tighter tolerances.

NIST only estimates the lifetime of M-Discs, real world abuse tests on BD's (non LTH, should have mentioned that to be honest) show good endurance that far exceeds DVDs. It comes down to however burning it right and storing it right. A pile of M-Disc left in a window in your uninsulated garage year after year and burned at 16x are not on the whole going to be in a better state in 20 years than a pile of BD-R's burned at 4x, stored in protective sleeves in a case in a temperature controlled, insulated environment. Add in having a back-up copy and the chances of total data failure on both primary and backup disc and you're looking at better survivability. NIST numbers generally assume things like storage in archival quality environments such as old salt mines which are a controlled environment, low humidity, neither excessively hot or cool and not subject to shifts in temperature. Most people can't store things in an environment like that and those who can usually have the finances for a better solution like multiple tape copies and/or continually updating and refreshing hashed/checksumed files and moving on a schedule to new better storage mediums (e.g. keeping files in a raid array in a plugged in NAS, checking for failures regularly, replacing disks and upgrading disks every 5-10 years one at a time).

I wouldn't trust any media not professionally stored in a purpose-built archival environment and with at least two copies to last more than 25 years without degradation or loss. Anyone trying to store stuff really long-term and cannot afford degradation or loss needs to have a plan to update their archival copies every 15 years or at least do an assessment that often and survey the options as well as the physical and ideally logical state of their chosen back-ups.