Overwhelming prior makes my claim more likely to be correct than the majority of claims made by myself or others. ;)

Very Bayesian of you! This is potentially confusing, though, in that you made a mathematical claim. Frequently mathematical claims mean that you have a proof of something, not that it's very likely. This issue comes up with computerized proofs in mathematics, like the four-color theorem. It's very likely to be true, and is usually considered proven, but we don't actually have a formal proof, only a computer-based one.

Note that your logic would apply equally well to Mersenne primes of N digits, for sufficiently large N. This makes sense in a Bayesian framework, but in a mathematical framework, you could take these statements and "prove" that there were a finite number of Mersenne primes. Mathematical proofs can combine in this way, though Bayesian statements of near-certainty can't. For instance, for any individual lottery ticket, it won't win the lottery, but I can't say that no ticket will win.

I think that the above only gives the odds that there are no such primes unless there is some good deep reason (presumably a set of symmetries, which doesn't seem at all likely since billion is an arbitrary seeming round decimal) for there to be some such prime or primes. Without that caveat, such statements would bite-in-the-ass far too many people historically who would have made overly confident mathematical claims. To clarify; I think you should be ridiculously confident, but not as confident as your reasoning by itself would justify.