I think in your example, as long as you have 8000 in the MCP altitude window, the VNAV descent will continue power on.
If, on the other hand, you had 9000 in the alt window, It would capture 9000 and, I think, would then calculate a new TOD which would be dependent on later alt constraints as well as the 8000' one.
I'm sure a VNAV expert will put me straight, then I'll get the books out and do some proper revision!