A theoretical lifespan of 1000 years is actually not that big. Factory-produced CD-ROMs which were replicated from a master-disk are expected to last for 100 years or more, but of course you cannot put your data on these. Next come gold-plated CD-R and DVD-R disks which are claimed to last for about 100-200 years by the manufacturer. Those claims are based on accelerated ageing tests, just like with M-Disks, so for me they sound just as valid. I still have my CD-Rs I have recorded 20 years ago, so the lifespan of regular CD-R disks is not 5 years like you said, unless perhaps you go for the cheapest ones. But if you really have found that disks which should last 100 years only last 5 in your environment, I would reasonably expect that a 1000-year lifespan disk should last about 50.
The real problem your descendants will likely to encounter in 100 years (let alone 1000 years) is to find the equipment to read the old disks you have left them. Typical CD and DVD drives are designed to last for 5 to 10 years of normal usage, and have perhaps 15 to 30 years of shelf life. It's hard to predict for how many more years CDs and DVDs will remain in use, but they will disappear eventually, and then your kinds will have a hard time reading those disks no matter how much you have paid for them.
Personally, I keep my data on a couple of hard drives, and copy it over to newer ones every 10 years or so. Sure, I'll need 200 HDDs over a thousand years, but I won't ever encounter troubles reading my backups on modern computers, and the capacity will keep growing over time, accommodating new data. If I decided to use M-disks instead, I'd have to buy new disks for new data ($30 for 50 GB on amazon) every year, so it would be more expensive, and my old M-disks would still age.