TBH, we shipped fully working strong consistency in 2014. It just had a limited feature set, was disabled by default, and was never promoted/marketed since it didn't fit the direction the new CEO/CTO was pushing.
The engineering exodus around that time sorta killed the project though, and we never were able to do the big follow-up work to make it really shine.
(Disclaimer: Former Basho Principal Engineer, primary author of strong consistency work, lead riak_core dev from 2011-2015)
I think another 18 months would have been enough too. But it just wasn't the right environment after the hostile take-over / leadership transition.