Having worked on embedded systems with (nearly) read-only root filesystems: An even bigger motivator is that flash memory lifetime is more determined by number

Having worked on embedded systems with (nearly) read-only root filesystems: An even bigger motivator is that flash memory lifetime is more determined by number of writes per block rather than time or reads per block. So to keep your storage both cheap and reliable, it's best to flash it with a single full (compressed) image every time you run a firmware upgrade, and otherwise mount it read-only.