Hey, when I put my f1-spirit cardridge in my msx, and startup, the game plays at 50 hertz.
In Japan the game would play at 60 hertz.
The monitors used in Japan are 60 hertz monitors, and the ones I use is 50 hertz.

Now my question is this: If a monitor would be designed to run at 70 or 80 hertz (and Im sure that exists), would the game then also run at 70 or 80 hertz !???

No. The monitor just displays, it doesn't decide the frequency. The 50/60 Hz are TV standards. PC's can do better and need better monitors to display those signals.

The VDP decides at what frequency the image is passed to the monitor. The V99x8 has one bit to flip between 50Hz and 60Hz, so it's not (only) a limitation of the monitor, but mainly a limitation in the VDP.

[edit]minor stuff[/edit]