@luz 'MTD and eraseblocks'
I wanted to say: maybe you should calculate the lifetime with eraseblocks (xx KBytes) instead of flash pages (256 Bytes) and you should take into account only a part of the chip.
So at least at the mtd layer, it is operating with 64kB (0x10000) erase blocks.
I've forgotten - I had already read this in the OpenWrt documentation. It seems I'm getting older... :( Maybe I should write everything down.