After FSX there seemed to be no other comparable flight games around
Remember that the MS Flight Simulator series (FSX being the last of that series and now carried on by Lockheed Martin as P3D) have never been intended as 'games' - although they do have that option. They were originally and throughout, designed as simulators - with the whole world there to be explored - and open to 3rd party addons to enhance the realism. remember that some payware FSX addons are so complex that they are used by commercial flying schools for training. And Lockheed Martin buying the rights of FSX from Microsoft kind of says it all.
I remember being given a copy of a very early edition of FS by a friend -- it was black and white and a very primative Meigs & Chicago that looked like it was all built with lego. (previously I had used an even more primative computer FS with a few instruments in the screen and a line as a runway on a black screen. That was way before windows came out)
Having said that -- it's still entertaining - but most 'gamers' will soon get bored by the lack of attacking aircraft, bullets, bombs, realistic explosive crashes and score progress to compare with yourself and others. They will go on and find a flying game - with many available in phone/tablet form these days.
Meanwhile that leaves the rest of us who actually like to pretend we are flying the real thing in FS. & there is actually no limit to the potential realism in FSX/FS2004 ..

Simplicity is the ultimate sophistication. – Leonardo da Vinci