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.