I read it a while back, it's not front of mind. I'm aware of libbitcoin.

I'm definitely aware of that criticism, and it's a difficult issue to square away. At a really basic level, the chain enforced by the majority of nodes is the "real" Bitcoin. Any new entrant needs to ensure that it won't go out of consensus. Onus is on the new implementation.

Optimal situation is probably only one implementation, from a consensus integrity perspective. But that leads to a different security risk. One bug could take everyone out.

Since multiple implementations exist, no one implementation being the majority is the most secure option.