Here's my reasoning on this. If MS could say that current generation WP phones will get Apollo, they would have good reason to do so. It would be good for sales, and thus it would help the OS to build momentum.
If they can't give this assurance, there are two possible reasons why. First, they may not know. Maybe WP8 just isn't in a finished enough state to be able to say. Second, they may know that the answer is negative, but announcing it would create the "lame duck" OS situation that RIM has right now with BBOS 7. This is bad for sales, so they have good reason not to do it.
Personally, my guess is that they know that current devices won't get WP8. I don't believe the OS is so far from ready that they can't say what hardware will be needed. In fact, even though I'm no software engineer, I'd go farther and say that WP8 was probably designed from the start to a particular hardware configuration. I can't really imagine why anyone would design an OS with the idea to see what it'll run on when it's done!
To me, this is the best explanation of why MS has persistently dodged this question. They know the answer is bad for business.
To all those who think MS "owes" them an update, there's simply no basis for such a belief. Nothing has been promised or even hinted, beyond backward compatibility for apps. The "understanding" that some people have about this is nothing more than wishful thinking.
Keeping early adopters happy is a very low priority in comparison to gaining new adopters. Suppose WP8 is terrific but can't run on current devices. If for every disgruntled person who leaves the platform they bring in two or three new adopters who weren't, that would be a great success.
Yes, we'd all love to get WP8 without buying new hardware. There's no doubt about that, but if WP8 needs better or different hardware to compete with the other players, then that's how it has to be.