I've been working in Storage, Archive, Backup and Recovery for about fifteen years. My background is UK financials, but I now work for one of the large Software/Hardware manufacturers.
I would say that optical is pretty much dead, no-one really uses it any more and the disks - unless you're talking super-expensive magneto-optical enterprise quality systems - only last about 5-10 years, and that's with good handling, in a climate controlled room.
If you have a lot of data and a lot of money, go for a tape robot, with a couple of the current generation of LTO drives, never use only one drive as you should really write on one and verify on another to make sure readability as sometimes drives will be fussy about tapes they will and won't read. Record the data on one tape and clone it to another, store one tape offsite at an offsite storage company. Chose your software wisely, it's all very good to have a tape system, but you need to have software to manage the data on those tapes. TAR on its will not do for anything above trivial use, archive software is typically expensive.
As you're currently using Drobo, I'm going to guess that the above isn't a goer, as it's really a low end of midrange solution and a Drobo is a workgroup device.
One of the currently available CAS (Content Addressed Storage) solutions would be a good, although probably too expensive solution. Then you have the problem of backup/protection of your CAS system and you're back to the initial problem. That said possibly you can replicate to a remote CAS system, this would be an acceptable solution. Again, this won't be cheap.
A filesystem which checks it's contents' integrity, such as zfs or refs is probably the way forward, however you are stuck with disk and an alterable filesystem, this is not really an archive. Personally this is pretty much what I use, my backup server's datastore is replicated to a disk, which is stored off-site during the week and replicated on Mondays, before being taken offsite again. By offsite, I mean my garage, which is about 600m from my house, so a nice distance away in case of fire/burglary etc. The nice thing about this solution is it's inexpensive and easily upgraded over time.
You could use a system like the above and replicate over DSL to a friend's house, however bandwidth issues will likely put a stop to this. Also, don't use anything like real-time replication, this is an excellent way to lose everything.
Reliability and expense are the key issues. Tape, such as LTO, will only read n-1 or n-2 generations, this means you're on an expensive upgrade cycle of tapes and drives every few years, sustainable for an Enterprise, but not a private individual. Tapes do occasionally snap, so you will need to keep two of each tape. They are also not a random access device, so can be slow.
Optical disks don't last, unless you're paying mega-bucks.
It would be nice to have something different, but I suspect your best option is good old fashioned spinning rust.