We covered this earlier monsterer, but I don't agree that observers can necessarily see the fault. If mining is centralized and no one outside of the collusion mines (because it is not economically viable to do so), then there will be no forks.

However, it is accurate to say that if we know there are miners who aren't part of a collusion and we don't see forks that exceed those accounted for by natural propagation, then there is no attack.

I believe the bolded condition is a near certainty today, and the italic condition is very likely true.

Therefore Bitcoin is solving (something like) BGP for the moment.

Analyzing the present based on available evidence is the only objective statement anyone can make on the matter. Speculations about the future are just that.