Just because the relational model has a very good track record, doesn't mean we shouldn't be careful what we take for granted. For example, if someone had told me even five years ago, that today I would be running more Linux machines in my home than Windows machines, I would have thought they were stark raving bonkers. Back then I had Linux (fedora) as a bit of a curiosity, dual booting on one machine along side XP. I had two other machines; a Vista and another XP. Windows outnumbered Linux 3:1, and my Linux install served no real purpose except mild amusement.
Today my family has between us:
:: 4 phones, all running Linux
:: A Galaxy tab, running Linux
:: A NAS, running Linux out of the box
:: A Vista machine
:: An XP machine (dual booting Ubuntu)
:: A Tivo running Linux (I'm pretty sure it's Linux)
:: An old box given a new lease of life running Ubuntu (still just a plaything).
Linux outnumbers Windows 9:2 and it just... happened. There was no conscious decision for much of this. I learned the other day than a large employer near here (13,000 EFT) is dropping MS Office for Google docs. Management's attitude is "the staff will just have to adapt". I wish them luck, sounds like my idea of hell.
The point is that these changes can sneak up on us, and presumably data management is not immune. The biggest user of organised data in the world would have to be Google. Is all/any of their data organised in the conventional relational way? Just as Windows missed (perhaps intentionally) the evolution of the OS into almost a disposable consumer item, RDBMSs could go the same way. We may not notice it at first because the RDBMSs reach may be still expanding, but if they fail to expand as rapidly as the "next big thing" they may eventually fade into the niche world.
...One of the symptoms of an approaching nervous breakdown is the belief that ones work is terribly important.... Bertrand Russell