The most unique thing about spanner is the use of globally synchronized clock timestamps to guarantee "comes before" consistency without the need to actually synchronize everything.
There is nothing stopping startups and open source developers from building the same thing in a few years. The missing ingredient is highly stable GPS and local time sources which will hopefully be available on cloud instances sometime soon. This is a new piece of hardware so it will be interesting to see if cloud providers make one available or use the opportunity to sell their own branded "service" version you can't buy. Unfortunately I think we'll see the latter far before the former, it it ever even exists. Without a highly stable timesource doing what spanner does will be completely impossible.
Yes spanner is special right now but that's even more reason to not go near it. Google has a complete monopoly on it, the strongest vendor lock in you can possibly have
Only "new" in the sense that it is currently not commonly offered, the devices themselves have been available for ages. (If you are a large enough customer you apparently can get at least some colo-facilities to provide you with the roof-access and cabling needed for the antennas). If cloud providers make precise time available I don't see much potential for locking you in with their specific way of providing it, as long as it ends up as precise system time in some way.
I know GPS time sources have been available forever but a fault tolent database needs a backup. The US GPS is incredibly reliable but there have been multiple issues with both Glonass and Galilio.
It sounds like Google has an additional time source making this possible, probably a highly miniaturized atomic clock, possibly on a single chip. There's no way they're running on GPS alone