Why do you think that hosted cochroachdb would be cheaper then hosted spanner? Google has been optimizing spanner performance for years so I would expect that it will be cheaper to run for quite a while. Of course the markup can be different but I wouldn't expect it to make a huge difference.
I just started playing with spanner. The API is nice. Simple txs are a bit slow - seconds instead of milliseconds - ok for my use-case. I don't hear much about it though, few articles on HN, I was wondering how mature it was and how widely used it is. Is it common knowledge that it is used by a lot of orgs?
De-throne in the sense of being the top-tier multi-master consistent transactional DB. I doubt too many people use Spanner outside of Google in reality. Google would likely eventually adopt cockroackdb if it was clearly better long term.
This database has the potential to dethrone Spanner in a major way.