I also hope to last decades, so to me there is a world of difference between media that lasts 30 or 40 years, and media that lasts 100 or 200 years. For better or worse, as long as we both avoid accidents, an M-disc will outlast me.
From my research into it some time ago - BDXL quad layer(128GB) BD-Rs are made basically in the same technology as M-Disc, so they should last hundreds of years, so that's all I've been using for my archival needs so far. Getting them from Amazon japan is pretty cost effective too.
