It's simplistic to say that the problem was "solved decades ago". Every existing attempt at Byzantine fault tolerance is limited in one way or another - if you're looking for a solution it's really a matter of choosing a solution based on which set of painful limitations you want to live with.

Bitcoin's solution is a pretty good one for its use case. Most other solutions would have trouble scaling like the blockchain can, but in many applications its very long consensus lead time would be unacceptable.

Correct.