Everything (natural) breaks down after time, at least most natural things do.
Rocks are natural, and yes and hundreds of thousands or millions of years they break down. We're not concerned about geologic time scales with regard to our aquariums however though are we? Well I know I'm not.
Yes all wood is natural, but that doesn't mean they all behave the same way, especially once submerged. Some woods rot fairly quickly under water, others more or less never rot if they stay submerged. (repeated wet and dry cycles are another thing altogether).
Your grand kids will use your cedar driftwood or roots, try that with manzanita. Around here we have lots of cedar stumps from the original clear cutting that was done in the 1850's. The outer few inches is rotting (but still there) but underneath the wood is still in good shape.
Most of woods would have rotted away to nothing long ago. That's not even submerged, that's from repeated wet/dry weathering cyles which is the hardest thing for a piece of wood to withstand. THAT is the difference between cedar and most other woods - so you can't lump everything that's "natural" into the same category and just say "well it all rots - same same" not the case. 5 years from now, and 90 years from now are two vastly different realities.
There are native american cedar totem poles which still stand.
There's an old cedar root bundle outside in the garden that I've been watching for 8 years (who knows how long it wast here before that) if I grab one of the thinner roots, say 3/4 of an inch, it doesn't give under pressure, I have to use some force to snap it. Fir, maple, oak, alder, it all would have very rotten by now.
In contact with the ground, a cedar post will start to rot after a time - submerged, you'd better be a vampire or you won't live to see it decay.
It's safe to boot, (so long as you use a seasoned piece) despite what gets regurgitated now and then on forums (by people who don't use it)