I am saying that in a decentralized, trustless, Sybil-attackable scenario, there is also no conditional solution to BGP, because the participants have no way to conjecture the probabilities of 51% attack
We'll have to agree to disagree. As long as I can write down a solution, and write down the condition under which it applies, then I consider that a conditional solution. I do not need to state a probability that such a condition will be satisfied.
What use is a condition if it can't be measured?
The entire construction is an exercise in theoretical computer science, i.e. mathematics. You state a problem and you solve it. In this case (as in many others), the solution has necessary conditions.
It also happens to have some practical applications. Some of those involve measuring or estimating probabilities, some may not. An example of the latter would be comparing two available solutions to the same problem, where the input probability is unknown, but one solution or the other must be chosen. In that case you would very likely choose the solution with the weaker necessary condition (or at least you would consider that against cost).
I'm not going to respond to the rest of your message because I think it basically comes down to you being convinced that economics will
certainly cause a permanent centralization of Bitcoin (which is effectively >1/3 or 50% collusion), and it will therefore fail. That's a fair belief, and I consider it a very significant probability, but I don't share your belief in the certainty of it.