To say they have absolutely no competitors isn't an accurate conjecture. I personally like PMDG's products very much, and speak of them highly. However, Flight Sim Labs is a developer whose systems modeling is near that of PMDG (their Concorde was an excellent acheivement for an airplane that hasn't flown in a decade), and the Airbus A3XX series will be an equal counterpart to the 737NGX, I'm sure. The thing is, there are other developers out there, and therefore they have competition; whether the others equal their abilities is of lesser importance, because others still exist.
Next, the timespan for FSX is by no means waning, to the contrary, new technologies are being developed everyday, and things are only getting better. A2A's new Accusim P-51 Mustan came out Sunday, and it is the sixth generation Accusim airplane; each release has produced an incrementlly better product, that does things previously thought untoutchable. Have you seen Orbx's new Stewart CZST Airport? It has 2cm per pixel of resolution on the airport grounds. They have trees that sway with the wind, snow that gathers on the rooftops, leaves that fall off as the days get shorter, and many other thing exist in their newest airports. The fact that Microsoft disbanded the Aces team was the greatest blessing yet given to us. Before, we'd have an new FS every two years, and yes there'd be new products, but seeing that a superior successor was just around the corner already, no new technologies were developed, nothing was heavilt researched, nothing got past the FS itself in being better. Now, it has been six years since FSX was released, and developers are introducing newer technologies, making the sim a more realistic product in all aspects: fligth dynamics, systems modeling, and the scenery. There is only a bright future ahead for FSX: PMDG will release two of the greatest widebodies soon, and a DC-7; Orbx will soon have out a Northern California, New England, and Great Britain, with a myriad of airports to go along (just this year!); A2A now has the Lockheed F-104 Starfighter on its books for Accusim, followed by the McDonnell Douglas F-4 Phantom II, and for the first time in years, their queue of Accusim in development as reached the pint where the exists only one more product past the current project, many new opportunities; Carenado keeps releasing their general aviation planes, although not the utmost in realism, they do the job, look very nice, and are great for those who want to get up and fly. As we look into the next five years, Orbx, PMDG, and others have already promised that they will continue to develop for at least this next half-decade, and by that time has been reached, I wouldn't be surprised if they keep developing. Now that computers exist that can handle FSX maxed-out (that are commercially available, and economically feasible), FSX is the only way to go, and is by no means waning, it is waxing!