I think that real AM DX'ers will take this change in stride. After all, who else will tolerate extreme amounts of interference, static and fading just to hear a station they've never "caught" before.
They will probably just note that "...station WXXX was received pre-2007..." or "...station KZZZ was received post-2007..." and keep on straining to catch the farthest station that they possibly can.
Even more, eventually many will begin DX'ing with HD receivers, which will open up another avenue of long-distance catches.
I certainly think so. We've tolerated a lot of things that our forefathers said would kill the hobby.
I think the other big question is, just how far will HD spread on AM? I don't think last week's FCC action is going to spur much additional adoption of HD - most of the stations that were going to go HD would have done so anyway. HD is still relatively rare on regional and local channels. If adoption doesn't go much beyond the big Class A clear-channel stations, then DXing on the regionals won't be much affected.
I have now heard that some engineers feel the widespread adoption of HD could kill not the DX hobby, but AM radio itself. Ironically, such a development would probably *improve* DXing (provided DXers were patient enough to wait it out) of the remaining stations outside the US. Personally, I think that's rather unlikely to happen.
I've done a *bit* of HD DXing. So far, only one catch: WHO Des Moines. (I'm near Nashville) I do think there will be a lot more DX logged once the new rules kick in and all-night operation is allowed.