I'm a big fan of single series PC racing sims, but I do think its worth remembering why they died out.
Sadly licensing racing series became something of a war zone. The F1 license, which at one time spawned games from Microprose/Crammond, EA/ISI and others, became a Playstation exclusive killing Crammond's team. NASCAR which also supported several developers over the years, became an EA exclusive killing Papyrus (even if it did rise again as iRacing).
From the ashes of the demise of PC based NASCAR and F1 sims rose Ian Bell's Simbin. With the two highest profile racing series licenses off limits they opted for a relatively obscure championship - the FIA GT Championship - which though obscure, featured high profile tracks like Spa and Monza combined with high profile cars like Porsche, Ferrari and Lamborghini. It proved a big hit.
Either because they wanted to diversify their portfolio of sims or because (unlike F1 or NASCAR) they didn't feel the market would go for annual updates on the GT championship, Simbin's next choice of racing series license was just as inspired - the FIA Historic racing championship. And GT Legends was born.
When Ian and most of the team left Simbin to form Blimey!, the new Simbin kept the rights to the FIA GT License and opted for the WTCC as their second string to their bow. But again the dangers of racing series licenses re-appeared and Porsche and Ferrari pulled out of the FIA GT license. This killed Simbin's planned successor to GTR2 and left them with little option but to offer just licensed cars and licensed tracks but no licensed racing series in the form of GTR Evo. Arguably this was the birth of racing sims as we see them now.
So while I remain a big fan of single racing series sims, there are dangers for developers who become reliant on them. Licenses can become exclusive (F1 and NASCAR). Race series can decline in popularity or cease to exist (Indycar, FIA Historic, FIA GT, WTCC). And if you don't have the option of the high profile championships such as F1 or NASCAR then finding that low profile championship with the right combination of tracks and cars with international appeal is a skill mixed perhaps with a bit of luck. Simbin seemed to get the FIA GT license at its peak, the FIA Historic license at its peak,and the WTCC at its peak (and subsequent decline). I think Kunos with Blancpain have got the best of the lower profile racing series; perhaps its at its peak or perhaps it will keep on its upward trend.