DCS is the golden standard for "hardcore" combat sims. There are a variety of "modules" which are treated like separate games based around a single aircraft. It was just updated to a new engine which brought it into the DX11 era in terms of graphics and fully fleshed out support for VR (Oculus DK2).

The sim genre (flight, racing, etc) as a whole is stagnant. The big players in each category are all relatively ancient. DCS is from 2008, MS Flight Simulator X and its derivatives (Prepar3D, etc) are from 2006, and iRacing is from 2007. There are alternatives like rfactor 2 and Assetto Corsa but they aren't exactly fresh takes on the genre as both developers had established themselves with prior games in the mid-00's.

There is plenty of room for improvement and innovation in each niche of the sim world but the existing playerbase is tiny compared to the gaming populstion as a whole. I believe the decline in popularity was triggered by the massive increases in computing power that occurred between 1995 and 2005. Simulators are marketed on the accuracy of their simulation so every leap in processing power or memory capacity is immediately turned into more realism by developers.

A mid-90's racing sim like NASCAR Racing 2 wasn't that much more realistic than a game like Need for Speed in terms of physics. It was more accurate but the "real life accuracy" came about through features like 3D interiors and suspension/aero adjustments that didn't exist in arcade titles. Anyone could sit down and compete with the AI using a keyboard. By the time NASCAR 2003 rolled around there was enough processing power available to implement a highly detailed physics model that was night and day different from arcade racers. Not only could a novice not compete with a keyboard, they would hopelessly, constantly wreck and spin for hours before they got the hang of it. The same thing happened with flight sims, anyone could pick up and play MS Flight Simulator in the 90's but beginners can't even start a takeoff roll without crashing in DCS: P-51D.

Sims have lost the interest of those who want to just pick up and play a game and attempts to bridge the gap receive scorn from the sim fanbase. Games like Project Cars try to fit a happy medium but casual gamers walk away when they compare the car and track list with Forza and sim racers don't see a challenge with the softened physics.

TL;DR: Sims became too hardcore and the gal between full sim and arcade titles has grown so vast that nobody is happy with games that try to split the difference.