Well, I see what you mean about FSX vs Flight, but if Flight runs better than FSX, I don't dee why anyone would stick with FSX for a long time, (even if they have purchased many add-ons). Right now it's simple : there is no such computer that can run FSX at ultra-high settings at major airports with add-ons and maintain a steady 30+ FPS, and that is why some still fly FS9. If you have the name of a computer that can do this, please send me a link! 
I have an i7 build, and still sly FS9, because even an i7 doesn't get good enough FPS on FSX for me. But I am basically echoing what you already said.
If FSX ran as good or better than FS9, no one would be using FS9 anymore, and we all know it. So, if Flight beats FSX performance, I don't see how FSX (much less FS9) can survive. But then again wouldn't that make life so much easier for Virtuali and the gang? 
I'm not going to lie, getting FSX to run perfectly takes more work than FS9 does with todays hardware. I mean you can take pretty much any newer computer, load FS9 and all your addons, set up your nVidia control panel, and go. Probably not much else needs to be done. FSX takes more work to set up well and get it running at high levels. In my case I am running it on an overclocked C2D at 3.85ghx, 2GB of RAM, and a 1GB OC GTS 250, far from high end compared to what is available now, however after enlisting the help of a 3rd party who specializes in FS9/FSX setups my rig runs it at what I would consider levels higher than FS9, but not completely maxed out and still manage sold FPS at major hubs with loads of AI, weather etc. What I mean when I say at levels higher than FS9 but not maxed FSX is that I can run larger amounts of autogen than what FS9 provides, higher res ground and cloud textures than FS9, 100% AI 99% of the time, some vehicle and boat traffic, etc, but not completely maxed out of what FSX can do. That being said, about the only airport that I have to tone AI down a bit is JFK and one or two of ORBX's major hubs when I am flying heavy metal. Doesn't really bother me though to trim the settings for two or three airports though since I dont live and die by KJFK or the few ORBX airports since I can still fly pretty much anywere else I want with a sim than in my opinion looks much better than FS9 does, primarly due to the ground and water textures.
I'll be frank though, the ground and water is the main reason I prefer FSX. I had FS9 on my rig up until a few months ago and while I tried to use it once in a while, after being so used to the ground/water in FSX I just couldn't enjoy it anymore, even for flights into JFK. Had I never spent so much time in FSX and got used to those ground and water textures then it would probably be a different story, but it is what it is. I'll pare back my settings if need to go to JFK, but for the majority of the rest of the world FSX with the PMDG birds and FSDT, FlyTampa, and Flightbeam sceneries work surprisingly well on what is now an antique computer.
Anyways, it will be interesting to how Flight measures up. I am optimistic that it will be an improvement, however with the track record MS has of always building these sims for the next generation of computers, it will be interesting to see if the next version will indeed run at high levels on todays hardware. If thats the case that it does, then maybe we will see the majority of the pack convert to one sim, however it's still going to take some time for our precious 3RD party sceneries and aircraft to show up to the party if backward compatibility is completely broken. If its not and FSX material can easliy be made to work then we might be all set from the start, however I have a feeling that it wont be that easy.
Say Flight come out this December for a holiday release and backward compatibilty is completely broken, it will probably be at least a year or more which would be some time in 2011 or early 2012 before new airports/aircarft start showing up. If that's the case there is no way I will even bother to get Flight until I can at least replace one or two of my favorite airliners like the 747, 737, or 767 and at least 10 or more aftermarket airports. Even then it I dont know if I could be enticed for such a small amount of content. On the other hand, say FSDT gets their hands on a copy of Flight and the SDK and finds out, hey this is simple to get FSX content ported to work in Flight and Flight is a much better sim, I will be on Flight quicker than a fly on poop if you know what I mean, lol. Hopefully that is the scenario, but it will probably be 6 months or more before we find out either way.
