By “not that long” I mean one to two years for a good quality SD card that is overprovisioned. For example, a high-quality SD card from a respected company that is 16G or 32G would last much much longer than a knockoff SD card that is 4G or 8G.
The wear happens on writes. There are only so many writes that each bit in the memory can perform. But the card does spread the writes across all the bits as evenly as possible so specific parts don’t wear out early while other parts remain never written to. So by over-provisioning the SD card you give it much more room to write to before it has to loop back around and write to the same bit again, giving you more time before the SD card wears out.
USB flash drives use the same technology, though again, the higher quality ones will probably last longer than the ultra cheap ones.
To me the big thing about Flash storage that makes it risky to use is not how long it lasts, it is that it fails silently. By the time you start to notice symptoms of a failing SD card or USB drive you have no idea how long it has been failing and therefore don’t really know how reliable your backups are or how far back you have to go to get to a guaranteed uncorrupted image.
So first of all the SD card doesn’t move files around to write level. Files that remain static (e.g. OS files) get written once and those bits get taken out of the calculation for wear leveling. So the 10000 writes would only apply to the amount of the storage that is left over.
Also, not all cards are all that smart about how they implement the wear leveling which can further cause some areas of memory to be written to more frequently and therefore wear out more quickly.
If you have a swap partition, that also needs to be taken into consideration for writes.
I do not know what it means when Toshiba says it supports 10000 write cycles. They can easily claim that number without any wear leveling at all which will kill the SD card super fast, or use a poor write leveling algorithm which will extend the life a little bit, or have a really good one and extend the life of the card a lot. Without better information about how/whether they wear level the number is all but meaningless.
Finally, one area of concern that can be falsely identified as a failing SD card is file system corruptions. When the RPi loses power without shutting down it is very easy for it to end up with a corrupted file system which can look a lot like a failing SD card.