Thanks to everyone who responded. I've done a lot of research on the internet and even according to the two vets I've talked to, there's a really good chance that even after travelling out of town and spending $$$$ on a scope (which will require an anesthetic - my guess is $1500 or so), there'll be no definitive diagnosis. Also, even if there's a diagnosis, it's unlikely that we'll want to go ahead and treat him given his age and other health issues.
I guess I was just venting, because the vet gave me the impression that she thought I was irresponsible for letting him suffer and I don't feel that being skinny and having a snotty nose is suffering. How he looks doesn't matter to me, or him. As I always say to people who ask me how old he is (grey muzzle, etc.), HE doesn't know he's old yet. I'm sure he'll let me know when the time comes.
He's tolerating the antibiotics well (no side effects, appetite still great) so we'll see how he is at the end of the week. Even though I'm not a real believer in homeopathics, I've ordered something called Sinu-Rite on the off chance that it will help, as it looks like antibiotics are only a temporary solution at best.