There are many advantages of PC gaming:
-Customization of your game experience. Not only in terms of parts for your PC, but also software as well as various gamepads, flight sticks, etc., which are popular and supported in terms of games. This allows for more genre types due to various input devices, from great RTS' to flight games.
-Better graphics and generally better experience on the technical side of things.
-Better gameplay experience with a mouse and keyboard usually.
-Mods.
-Better deals (looking at you steam).
-More variety of games. We can argue to the moon and back about this, but I would be right in the end. On the PC, not only do we get dumb action games, but also smarter, slower games like Arma 2 or Stalker, Risen (360 version doesn't count because it's just BAD).
-Ability to play all games ever released (mostly). A far cry from the little backwards compatibility of consoles.
-Emulators.
-Cheaper than console gaming in the long run. It can be more expensive or the same price, but that depends on how you like to game on PC. If you spend money on the latest hardware every year, then yes, it's going to be more expensive. If you wait for deals and get the most out of your hardware, it will be cheaper. Consoles have renting, but with PC gaming games are cheap and you get to own everything you pay for. If you want to rent, consoles are a better choice, but for the gaming enthusiast who likes to own all his games, PC is more preferrable.
-Best online support, better community (in terms of content and various forms of communication, not necessarilly in terms of maturity, although I would argue that PC gamers are generally more mature), better clan support, etc.
-Better for smaller developers.
-More money per game for developers.
-Insert other advantage here.
Disadvantage:
-Needs tinkering with and involvement. Generally you need to know a bit about computers, which I don't think PC devs handle right. What I would do is have a third party that promotes PC gaming by putting a disc/file in every physical/digital copy that covers all the steps of installing various hardware and what various hardware is. This means explaining what RAM is, what a CPU is, etc, the advantages of building your own rig, so that if one were to ever want to progress further into the more technical aspect of PC gaming and get the most out of it, they would have a great starting point. The PCGA should do this, but sadly, they're useless.
-Needs work on community aspect. Involving modders and having great online support is essential.
Market aspects:
PC games don't sell as well at launch. However, they sell better than consoles in the long run. This is just an aspect of the market. The only disadvantage is that it's not good for churning out sequel after sequel. A successful PC game (built with PC's in mind) will do better than 99% of console games because the demand is great for PC-focused games. So generally, if you make a great PC game, you can be sure that the market will usually respond well and your game will sell great and continue to sell great years after release, as the PC is mostly a very stable market with stable demand (mostly) for hardcore games, but PC users don't rush to buy on day 1, at least not the hardcore ones.
Usually you'll sell around 2-3 million in the first month if you're Blizzard for instance, and sell 10+ million over 5-10 years.
Piracy shows interest for your product. The more successful the game the more piracy (Starcraft), so it usually isn't a problem. Piracy may also indicate issues with your game or its distribution (piracy of Valve games in Russia, for instance).
PC mainstream gaming isn't dying, it's just that the developers that now make multiplatform games are being replaced by smaller and independent developers who are slowly growing in size and so, PC has become the breeding ground for a whole new generation of developers (in comparison to the LAUGHABLE market for smaller developers on consoles) that will supply future multiplatform and big budget games. You may see Frictional Games as the next Resident Evil type developer, while CD Projekt RED and GSC (The Witcher and Stalker) are two examples of developers that have already become very recognizable names in the PC industry.
These are my thoughts and observations.