Light travels 186 miles per ms and a company like Google can put machines all over the place. The speed of light is really not an issue compared to other network overheads which can be improved.
So... Stadia, along with all other current and future streaming services, are going to really suck for a lot of people.
But... they are going to be really amazing for a lot of people too.
Best-case latency should be as low or lower than the latency many play with on their consoles.