Flight Simulator 2004
As a user of Flight Simulator 2004 who refused to switch to Flight Simulator X until I could produce the type of positive flying experience I had come to expect with FS9, I can appreciate the large number of users that are still enjoying their Flight Simulator 2004 (FS9) installations.
After all, for those still using Flight Simulator 9 there are tons of available add-ons and expansions. Many high quality commercial addons, some of which are at reduced cost because of their age. The cost of the actual Flight Simulator software is very good, often less than $20.
One anecdotal explanation I often hear from users of Flight Simulator 2004 and why they have not switced to Flight Simulator X is the large collection of both free and commercial software that they have amassed for Flight Simulator 2004. Addons that they still enjoy thoroughly.
These users also tell me that they are very happy with the performance vs. quality trade-off that they achieve with Flight Simulator 2004 (FS9) and they have not been able to strike the same balance in Flight Simulator X. Though one might think this is directly related to the age or quality of their computer hardware, that is not always the case. In some situations, the user has very reasonable hardware for running Flight Simulator X, but for some reason is just not able to run it without disruptive performance problems like Stuttering, or crash to desktops.
Flight Simulator X
As a user that used Flight Simulator 9 happily for nearly 1.5 years after the release of Flight Simulator X but who switched when I was able to produce the type of high quality flight simulator experience I desire, I can appreciate those that have made the switch.
Once I was able to get past the basic quality and performance problems I was experiencing with Flight Simulator X, I was able to finally see the very good qualities of the flight simulator and start to enjoy it fully. Although I waited nearly 1.5 years to switch to Flight Simulator X, the switch was fast and I went from 100% Flight Simulator 2004 to being a 100% Flight Simulator X user in about 2 months.
Lately, I have been using my Flight Simulator 2004 installation to feed some automated screenshot images to the Hovercontrol Multiplayer Map Page (Here), and because I have been using Flight Simulator X for so long now, I am able to clearly see some of the qualities of Flight Simulator 9 that I do not miss.
The weather environment quality, and the resolution of the ground textures are probably the two areas of Flight Simulator X that would be the hardest to give up now. Many of the changes in Flight Simulator X were not so much revolutionary as they were evolutionary, so at first you might not recognize all of the subtle improvements, but after a few months of using Flight Simulator X, and then returning to Flight Simulator 2004...these subtle changes become more obvious.
The rub for developers
I think developers, especially those interested in producing free software for the flight simulator community really want to support Flight Simulator 9 and Flight Simulator X. However, certain aspects of the Flight Simulator development process make that very difficult to achieve.
When there are only so many hours in the day, you can appreciate that a developer really needs to decide where it is best to spend those hours. I think those developers that are still focused on Flight Simulator 2004 are probably looking to take advantage of processes and tools that they had gotten working quite well over of the lifetime of the Flight Simulator 2004 product.
I don't think that any new developer would specifically focus on developing for Flight Simulator 9 however. In fact, I would think that if any new developers join the flight simulator hobby in the next 2 years, that they will have no prior experience with Flight Simulator 2004 to leverage. All they will know will be Flight Simulator X.
Flight Simulator X vs. Flight Simulator 2004
As always, each person has to decide which is best for them. If good performance on average hardware and the high availability of existing addons at no cost or low prices is a motivating factor, then Flight Simulator 2004 (FS9) might be the right choice for you.
However, if you intend to participate in the next 3-5 years of Flight Simulator community activities, or you intend to take advantage of new advances happening with the community and with various Flight Simulator products you can bet that many free and commercial developers will be focusing on Flight Simulator X as their base platform. If this describes your use of Flight Simulator, than it is probably best to choose Flight Simulator X as your primary base flight simulator platform.
In any case, enjoy your flying, and enjoy your hobby.