I was uncertain about this as well, but wouldn't the electron-depletion in the substrate be (slowly) replaced from the ground?

In any case, the charge on the device would be negligible in any direction. It's a ridiculously small amount of charge when compared to the overall device, or even the substrate material of the flash memory cells.

Would be fun to do an experiment though. Maybe in 100 years we'll be able to do that.