The true decline of PC gaming and video games in general, came with the massive popularization, the rise of multiplayer, the casualization, the transition towards digital format and finally, the change towards videogames as a service instead than as a product. All the dlcs, microtransactions, dumbing down, over-hype, pre-release purchases, youtubers, twitchers, mobile gaming, etc are consequences of that big paradigm shift in which a niche hobby was transofrmed in the first entertainment in the world.
Two graphs to iilustrate:
Even in a supposedly positive perspective, as the growth of titles number it's possible to see the huge change in the industry that happened around 2005-2006. The start of the decline match the exact moment in which for fist time in History "other platforms" -first mostly consoles and handhelds, later also mobiles and tablets- started to produce many more titles than PC, because that's an indicative of how casualized is the primary target for developers.
This recently updated graph from mobygames database shows this perfectly, with the dominance of consoles and start of mobile growth in mid 2000s, the appex of console-mobile dominance around 2012-2013, when thanks to indie boom, PC creativity recovered greatly, but the extreme growth of mobile market didn't allow it to regain its primacy.
In Crpgs case there was a clear first decline in 1994-1997 despite Realms of Arkania II, Daggerfall and Fallout, with not much more in 4 years -Diablo, Ultima 8, TES: Arena or Ravenloft-. Also those years in mid 90s supposed a brake or absolute final of many foundational series: Wizardry, Ultima, Gold Box, M&M and the transformation of many of the companies that led Golden Age.
The second great decline in crpgs started in 2005, and that year was probably among the worst for crpgs in entire History. In the next years were released the most terrible sequels and crpg genre was dominated by consoles times and style. Since 2012-2014 creativity returned... more or less, because the worst years in this new period are similar or inferior in quality terms than the best years of the "decline" period, so the new incline is fairly unstable. So if we compare:
2012: Legend of Grimrock, Tales of Maj' Eyal, Dark Souls port, FTL
2013: Expedition: Conquistador, Shadowrun: Returns
2016: Dragon's Dogma port, Fallout 1.5, Dungeon Rats
vs.
2007 Mask of the Betrayer, The Witcher 1, 7.62 High Calibre, Eschalon, Mass Effect
2009 Knights of the Chalice, Risen, Dragon Age: Origins
2010 Alpha Protocol, New Vegas, Mount & Blade: Warband
And in the case of 2019 I think is the most mediocre year since 2005 and I'm not sure if superior to 2006 or 2008...